The Project Gutenberg eBook, Airplane Photography, by Herbert Eugene Ives

AIRPLANE PHOTOGRAPHY

AIRPLANE PHOTOGRAPHY

BY

HERBERT E. IVES

MAJOR, AVIATION SECTION, SIGNAL OFFICERS RESERVE CORPS, UNITED STATES ARMY; LATELY OFFICER IN CHARGE OF EXPERIMENTAL DEPARTMENT, PHOTOGRAPHIC BRANCH, AIR SERVICE

208 ILLUSTRATIONS

PHILADELPHIA AND LONDON

J. B. LIPPINCOTT COMPANY

COPYRIGHT, 1920, BY J. B. LIPPINCOTT COMPANY

PRINTED BY J. B. LIPPINCOTT COMPANY

AT THE WASHINGTON SQUARE PRESS

PHILADELPHIA, U. S. A.

TO MY WIFE

A HELPFUL CRITIC, EVEN THOUGH SHE NEITHER PHOTOGRAPHS NOR FLIES

PREFACE

Airplane photography had its birth, and passed through a period of feverish development, in the Great War. Probably to many minds it figures as a purely military activity. Such need not be the case, for the application of aerial photography to mapping and other peace-time problems promises soon to quite overshadow its military origin. It has therefore been the writer's endeavor to treat the subject as far as possible as a problem of scientific photography, emphasizing those general principles which will apply no matter what may be the purpose of making photographs from the air. It is of course inevitable that whoever at the present time attempts a treatise on this newest kind of photography must draw much of his material from war-time experience. If, for this reason, the problems and illustrations of this book are predominantly military, it may be remembered that the demands of war are far more severe than those of peace; and hence the presumption is that an account of how photography has been made successful in the military plane will serve as an excellent guide to meeting the peace-time problems of the near future.

It is assumed that the reader is already fairly conversant with ordinary photography. Considerable space has indeed been devoted to a discussion of the fundamentals of photography, and to scientific methods of study, test, and specification. This has been done because aerial photography strains to the utmost the capacity of the photographic process, and it is necessary that the most advanced methods be understood by those who would secure the best results or contribute to future progress. No pretence is made that the book is an aerial photographic encyclopædia; it is not a manual of instructions; nor is its appeal so popular as it would be were the majority of the illustrations striking aerial photographs of war subjects. It is hoped that the middle course steered has produced a volume which will be informative and inspirational to those who are seriously interested either in the practice of aerial photography or in its development.

The writer is deeply in debt to many people, whose assistance of one sort or another has made this book possible. First of all should be mentioned those officers of the English, French and Italian armies through whose courtesy it is that he can speak at first hand of the photographic practices in these armies at the front. It is due to Lieutenant Colonel R. A. Millikan that the experimental work of which the writer has had charge was initiated in the United States Air Service. To him and to Major C. E. Mendenhall, under whom the work was organized in the Science and Research Division of the Signal Corps, are owing the writer's thanks for the opportunities and support given by them. A similar acknowledgment is made to Lieutenant Colonel J. S. Sullivan, Chief of the Photographic Branch of the Army Air Service, for his interest and encouragement in the compilation of this work, and for the permission accorded to use the air service photographs and drawings which form the majority of the illustrations.

The greatest debt of all, however, is to those officers who have formed the staff of the Experimental Department. To mention them by name: Captain C. A. Proctor, who was charged with our foreign liaison, and who acted as deputy chief during the writer's absence overseas; Captain A. K. Chapman, in charge of the work on optical parts, and later chief of our Rochester Branch; Captain E. F. Kingsbury, who had immediate charge of camera development; Lieutenant J. B. Brinsmade and Mr. R. P. Wentworth, who handled the experimental work on camera mountings and installation; Lieutenant A. H. Nietz, in charge of the Langley Field Laboratory of the Experimental Department; Mr. R. B. Wilsey and Lieutenant J. M. Hammond, who, with Lieutenant Nietz, carried on the experimental work on sensitized materials. A large part of what is new and what is ascribed in the following chapters to “The American Air Service” is the work of this group of men. Were individual references made, in place of this general and inclusive one, their names would thickly sprinkle these pages. It has been a rare privilege to have associates so able, enthusiastic, and loyal.

THE AUTHOR

November, 1919

CONTENTS

I
INTRODUCTORY

AIRPLANE PHOTOGRAPHY

CHAPTER I
GENERAL SURVEY

Aerial Photography from Balloons and Kites.—Photography from the air had been developed and used to a limited extent before the Great War, but with very few exceptions the work was done from kites, from balloons, and from dirigibles. Aerial photographs of European cities had figured to a small extent in the illustration of guidebooks, and some aerial photographic maps of cities had been made, notably by the Italian dirigible balloon service. Kites had been employed with success to carry cameras for photographing such objects as active volcanoes, whose phenomena could be observed with unique advantage from the air, and whose location was usually far from balloon or dirigible facilities.

As a result of this pre-war work we had achieved some knowledge of real scientific value as to photographic conditions from the air. Notable among these discoveries was the existence of a veil of haze over the landscape when seen from high altitudes, and the consequent need for sensitive emulsions of considerable contrast, and for color-sensitive plates to be used with color filters.

The development of aerial photography would probably however have advanced but little had it depended merely on the balloon or the kite. As camera carriers their limitations are serious. The kite and the captive balloon cannot navigate from place to place in such a way as to permit the rapid or continuous photography of extended areas. The kite suffers because the camera it supports must be manipulated either from the ground or else by some elaborate mechanism, both for pointing and for handling the exposing and plate changing devices. The free balloon is at the mercy of the winds both as to its direction and its speed of travel. The dirigible balloon, as it now exists after its development during the war, is, it is true, not subject to the shortcomings just mentioned. Indeed, in many ways it is perhaps superior to the airplane for photographic purposes, since it affords more space for camera and accessories, and is freer from vibration. It is capable also of much slower motion, and can travel with less danger over forests and inaccessible areas where engine failure would force a plane down to probable disaster. But the smaller types as at present built are not designed to fly so high as the airplane, and the dirigibles, both large and small, are far more expensive in space and maintenance than the plane. For this one reason especially they are not likely to be the most used camera carriers of the aerial photographer of the future. Inasmuch as the photographic problems of the plane are more difficult than those of the dirigible and at the same time broader, the subject matter of this book applies with equal force to photographic procedure for dirigibles.

Development of Airplane Photography in the Great War.—The airplane has totally changed the nature of warfare. It has almost eliminated the element of surprise, by rendering impossible that secrecy which formerly protected the accumulation of stores, or the gathering of forces for the attack, a flanking movement or a “strategic retreat.” To the side having command of the air the plans and activities of the enemy are an open book. It is true that more is heard of combats between planes than of the routine task of collecting information, and the public mind is more apt to be impressed by the fighting and bombing aspects of aerial warfare. Nevertheless, the fact remains that the chief use of the airplane in war is reconnaissance. The airplane is “the eye of the army.”

In the early days of the war, observation was visual. It was the task of the observer in the plane to sketch the outlines of trenches, to count the vehicles in a transport train, to estimate the numbers of marching men, to record the guns in an artillery emplacement and to form an idea of their size. But the capacity of the eye for including and studying all the objects in a large area, particularly when moving at high speed, was soon found to be quite too small to properly utilize the time and opportunities available in the air. Moreover, the constant watching of the sky for the “Hun in the sun” distracted the observer time and time again from attention to the earth below. Very early in the war, therefore, men's minds turned to photography. The all-seeing and recording eye of the camera took the place of the observer in every kind of work except artillery fire control and similar problems which require immediate communication between plane and earth.

The volume of work done by the photographic sections of the military air service steadily increased until toward the end of the war it became truly enormous. The aerial negatives made per month in the British service alone mounted into the scores of thousands, and the prints distributed in the same period numbered in the neighborhood of a million. The task of interpreting aerial photographs became a highly specialized study. An entirely new activity—that of making photographic mosaic maps and of maintaining them correct from day to day—usurped first place among topographic problems. By the close of the war scarcely a single military operation was undertaken without the preliminary of aerial photographic information. Photography was depended on to discover the objectives for artillery and bombing, and to record the results of the subsequent “shoots” and bomb explosions. The exact configurations of front, second, third line and communicating trenches, the machine gun and mortar positions, the “pill boxes,” the organized shell holes, the listening posts, and the barbed wire, were all revealed, studied and attacked entirely on the evidence of the airplane camera. Toward the end of the war important troop movements were possible only under the cover of darkness, while the development of high intensity flashlights threatened to expose even these to pitiless publicity.

Limitations to Airplane Photography Set by War Conditions.—The ability of the pilot to take the modern high-powered plane over any chosen point at any desired altitude in almost any condition of wind or weather gives to the plane an essential advantage over the photographic kites and balloons of pre-war days. There are, however, certain disadvantages in the use of the plane which must be overcome in the design of the photographic apparatus and in the method of its use. Some few of these disadvantages are inherent in the plane itself; for instance, the necessity for high speed in order to remain in the air, and the vibration due to the constantly running engine. Others are peculiar to war conditions, and their elimination in planes for peace-time photography will give great opportunities for the development of aerial photography as a science.

Chief among the war-time limitations is that of economy of space and weight. A war plane must carry a certain equipment of guns, radio-telegraphic apparatus and other instruments, all of which must be readily accessible. Many planes have duplicate controls in the rear cockpit to enable the observer to bring the plane to earth in case of accident to the pilot. Armament and controls demand space which must be subtracted from quarters already cramped, so that in most designs of planes the photographic outfit must be accommodated in locations and spaces wretchedly inadequate for it. Economy in weight is pushed to the last extreme, for every ounce saved means increased ceiling and radius of action, a greater bombing load, more ammunition, or fuel for a longer flight. Hence comes the constant pressure to limit the weight of photographic and other apparatus, even though the tasks required of the camera constantly call for larger rather than smaller equipment.

To another military necessity is due in great measure the forced development of aerial photographic apparatus in the direction of automatic operation. The practice of entrusting the actual taking of the pictures to observers with no photographic knowledge, whose function was merely to “press the button” at the proper time, necessitated cameras as simple in operation as possible. The multiplicity of tasks assigned to the observer, and in particular the ever vital one of watching for enemy aircraft, made the development of largely or wholly automatic cameras the war-time ideal of all aerial photographic services. Whether the freeing of the observer from other tasks will relegate the necessarily complex and expensive automatic camera to strictly military use remains to be seen.

CHAPTER II
THE AIRPLANE CONSIDERED AS A CAMERA PLATFORM

An essential part of the equipment of either the aerial photographer or the designer of aerial photographic apparatus is a working knowledge of the principles and construction of the airplane, and considerable actual experience in the air. Conditions and requirements in the flying plane are far different from those of the shop bench or photographic studio. As a preliminary to undertaking any work on airplane instruments a good text-book on the principles of flight should be studied. Such general ideas as are necessary for understanding the purely photographic problems are, however, outlined in the next paragraphs.

Fig. 1.—The elements of the plane.

Construction of the Airplane.—The modern airplane (Fig. [1]) consists of one or more planes, much longer across than in the direction of flight (aspect ratio). These are inclined slightly upward toward the direction of travel, and their rapid motion through the air, due to the pull of the propeller driven by the motor, causes them to rise from the earth, carrying the fuselage or body of the airplane. In the fuselage are carried the pilot, observer, and any other load. Wheels below the fuselage forming the under-carriage or landing gear serve to support the body when running along the ground in taking off or landing. The pilot, sitting in one of the cockpits, has in front of him the controls, by means of which the motion of the plane is guided (Figs. [2] and [3]). These consist of the engine controls—the contacts for the ignition, the throttle, the oil and gasoline supply, air pressure, etc., and the steering controls—the rudder bar, the stick and the stabilizer control. The rudder bar, operated by the feet, controls both the rudder of the plane, which turns the plane to right or left in the air, and the tail skid, for steering on the ground. The stick is a vertical column in front of the pilot which, when pushed forward or back, depresses or raises the elevator and makes the machine dive or climb. Thrown to either side it operates the ailerons or wing tips, which cause the plane to roll about its fore and aft axis. The stabilizer control is usually a wheel at the side of the cockpit, whose turning varies the angle of incidence of the small stabilizing plane in front of the elevator, to correct the balance of the plane under different conditions of loading.

Fig. 2.—Forward cockpit of DeHaviland 4, showing instrument board.

Fig. 3.—Rear cockpit of DeHaviland 4, showing rear “stick” and rudder bar.

The fuselage consists usually of a light hollow framework of spruce or ash, divided into a series of bays or compartments by upright members, connecting the longerons, which are the four corner members, running fore and aft, of the plane. Diagonally across the sides and faces of these bays are stretched taut piano wires, and the whole structure is covered with canvas or linen fabric. Cross-wires and fabric are omitted from the top of one or more bays to permit their being used as cockpits for pilot and observer. In later designs of planes the wire and fabric construction has been superseded by ply-wood veneer, thereby strengthening the fuselage so that many of the diagonal bracing wires on the inside are dispensed with. This greatly increases the accessibility of the spaces in which cameras and other apparatus must be carried.

Fig. 4.—Biplane in flight.

The fuselage differs greatly in cross-section shape and in roominess according to the type of engine. In the majority of English and American planes, with their vertical cylinder or V type engines, the fuselage is narrow and rectangular in cross-section. In many French planes, radial or rotary engines are used and the fuselage is correspondingly almost circular, and so is much more spacious than the English and American planes of similar power. The shape and size of the plane body has an important bearing on the question of camera installation.

Fig. 5.—A single-seater.

Types of Planes.—The most common type of plane is the biplane (Fig. [4]), with its two planes, connected by struts and wires, set not directly over each other, but staggered, usually with the upper plane leading. Monoplanes were in favor in the early days of aviation, and triplanes have been used to some extent. According to the position of the propeller planes are classified as tractors or pushers, tractors being at present the more common form. Planes are further classified as single-seaters (Fig. [5]), two-seaters, and three-seaters. These motor and passenger methods of classification are now proving inadequate with the rapid development of planes carrying two, three, and even more motors, divided between pusher and tractor operation, and carrying increasingly large numbers of passengers. Aside from structure, planes may be further classified according to their uses, as scout, combat, reconnaissance, bombing, etc. Planes equipped with floats or pontoons for alighting on the water are called seaplanes (Fig. [182]), and those in which the fuselage is boat-shaped, to permit of floating directly on the water, are flying boats (Fig. [183]).

The Plane in the Air.—The first flight of the photographic observer or of the instrument expert who is to work upon airplane instruments is very profitably made as a “joy ride,” to familiarize him with conditions in the air. His experience will be somewhat as follows:

The plane is brought out of the hangar, carefully gone over by the mechanics, and the engine “warmed up.” The pilot minutely inspects all parts of the “ship,” then climbs up into the front cockpit. He wears helmet and goggles, and if the weather is cold or if he expects to fly high, a heavy wool-lined coat or suit, with thick gloves and moccasins, or an electrically heated suit. The passenger, likewise attired, climbs into the rear cockpit and straps himself into the seat. He finds himself sitting rather low down, with the sides of the cockpit nearly on a level with his eyes. To either side of his knees and feet are taut wires leading from the controls to the elevator, stabilizer, tail skid and rudder. If the machine is dual control, the stick is between his knees, the rudder bar before his feet. None of these must he let his body touch, so in the ordinary two-seater his quarters are badly cramped.

At the word “contact” the mechanics swing the propeller, and, sometimes only after several trials, the motor starts, with a roar and a rush of wind in the passenger's face. After a moment's slow running it is speeded up, the intermittent roar becomes a continuous note, the plane shakes and strains, while the mechanics hold down the tail to prevent a premature take-off. When the engine is properly warmed up it is throttled to a low speed, the chocks under the wheels are removed, the mechanics hold one end of the lower wing so that the plane swings around toward the field. It then “taxis” out to a favorable position facing into the wind with a clear stretch of field before it. After a careful look around to see that no other planes are landing, taking off, or in the air near by, the pilot opens out the engine, the roar increases its pitch, the plane moves slowly along the ground, gathers speed and rises smoothly into the air. Near the ground the air is apt to be “bumpy,” the plane may drop or rise abruptly, or tilt to either side. The pilot instantly corrects these deviations, and the plane continues to climb until steadier air is reached.

At first the passenger's chief impressions are apt to be the deafening noise of the motor, the heavy vibration, the terrific wind in his face. If he raises his hand above the edge of the cockpit he realizes the magnitude of wind resistance at the speed of the plane, and hence the importance of the stream-line section of all struts and projecting parts.

When he reaches the desired altitude the pilot levels off the plane and ceases to climb. Now his task is to maintain the plane on an even keel by means of the controls, correcting as soon as he notes it, any tendency to “pitch,” to “roll” or to “yaw” off the course. The resultant path is one which approximates to level straight flying to a degree conditioned by the steadiness of the air and the skill of the pilot. If he is not skilful or quick in his reactions he may keep the plane on its level course only by alternately climbing and gliding, by flying with first one wing down and then the other, by pointing to the right and then to the left. The skilled photographic pilot will hold a plane level in both directions to within a few degrees, but he will do this easily only if the plane is properly balanced. If the load on the plane is such as to move the center of gravity too far forward with respect to the center of lift the plane will be nose-heavy, if the load is too far back it will be tail-heavy. Either of these conditions can be corrected, at some cost in efficiency, by changing the inclination of the stabilizer. When the plane reaches high altitudes in rare air, where it can go no further, it is said to have reached its ceiling. It here travels level only by pointing its wings upward in the climbing position, so that the fuselage is no longer parallel to the direction of flight. An understanding of these peculiarities of the plane in flight is of prime importance in photographic map making, where the camera should be accurately vertical at all times.

The direction and velocity of the wind must be carefully considered by the pilot in making any predetermined course or objective. The progress of the plane due to the pull of the propeller is primarily with reference to the air. If this is in motion the plane's ground speed and direction will be altered accordingly. In flying with or against the wind the ground speed is the sum or difference, respectively, of the plane's air speed (determined by an air speed indicator) and the speed of the wind. If the predetermined course lies more or less across the wind the plane must be pointed into the wind, in which case its travel, with respect to the earth, is not in the line of its fore and aft axis. The effect of “crabbing,” as it is called, on photographic calculations is discussed later (Figs. [136] and [138]).

When the plane has reached the end of its straight course and starts to turn, its level position is for the moment entirely given up in the operation of banking (Fig. [6]). Just as the tracks on the curve of a railroad are raised on the outer side to oppose the tendency of the train to slip outward, so the plane must be tilted, by means of the ailerons, toward the inside of the turn. A point to be clearly kept in mind about a bank is that if correctly made a plumb line inside the fuselage will continue to hang vertical with respect to the floor of the plane, and not with respect to the earth, for the force acting on it is the combination of gravity and the acceleration outward due to the turn. Only some form of gyroscopically controlled pointer, keeping its direction in space, will indicate the inclination of the plane with respect to the true vertical. If the banking is insufficient the plane will side slip outward or skid; if too great, it will side slip inward.

Fig. 6.—Banking.

As part of the “joy ride” the pilot may do a few “stunts,” such as a “stall,” a “loop,” a “tail spin,” or an “Immelman.” From the photographic standpoint these are of interest in so far as they bear on the question of holding the camera in place in the plane. The thing to be noted here is that (particularly in the loop), if these maneuvers are properly performed, there is little tendency toward relative motion between plane and apparatus. In a perfect loop it would, for instance, be unnecessary, due to the centrifugal force outward, for the observer to strap himself in. It is, however, unwise to place implicit confidence in the perfection of the pilot's aerial gymnastics. No apparatus should be left entirely free, although, for the reason given, comparatively light fastenings are usually sufficient.

When nearing the landing field the pilot will throttle down the engine and commence to glide. If he is at a considerable altitude he may come down a large part of the distance in a rapid spiral. As the earth is approached the air pressure increases rapidly, and the passenger, if correctly instructed, will open his mouth and swallow frequently to equalize the air pressure on his ear drums. Just before the ground is reached the plane is leveled off, it loses speed, and, if the landing is perfect, touches and runs along the ground without bouncing or bumping. Frequently, however, the impact of the tail is sufficiently hard to cause it to bump badly, with a consequent considerable danger to apparatus of any weight or delicacy. This is especially apt to occur in hastily chosen and poorly leveled fields such as must often be utilized in war or in cross-country flying.

Appearance of the Earth from the Plane.—The view from the ordinary two-seater is greatly restricted by the engine in front and by the planes to either side and below (Figs. [7], [8], and [9]). By craning his neck over the side, or by looking down through an opening in the floor, the passenger has an opportunity to learn the general appearance of the subject he is later to devote his attention to photographing. Perhaps the most striking impression he receives will be that of the flatness of the earth, both in the sense of absence of relief and in the sense of absence of extremes of light and shade. The absence of relief is due to the fact that at ordinary flying heights the elevations of natural objects are too small for the natural separation of the eyes to give any stereoscopic effect. The absence of extremes of light and shade is in part due to the fact that the natural surfaces of earth, grass and forest present no great range of brightness; in part to the small relative areas of the parts in shadow; in considerable part to the layer of atmospheric haze which lies as an illuminated veil between the observer and the earth at altitudes of 2000 meters and over (Figs. [10] and [11]). Due to the combination of these factors the earth below presents the appearance of a delicate pastel.

As the gaze is directed away from the territory directly below, the thickness of atmosphere to be pierced rapidly increases, until toward the horizon (which lies level with the observer here as on the ground) all detail is apt to be obliterated to such an extent that only on very clear days can the horizon itself be definitely found or be distinguished from low lying haze or clouds (Fig. [4]).

Fig. 7.—The view ahead.

Fig. 8.—The view astern.

Airplane Instruments.—Mounted on boards in front of the pilot and observer are various instruments to indicate the performance of engine and plane (Fig. [2]). Those of interest to the photographic observer are the compass, the altimeter, the air speed indicator, the inclinometers.

Fig. 9.—The view between the wings.

Fig. 10.—Appearance of the earth from a low altitude—3000 feet or less.

The compass is usually a special airplane compass, with its “card” immersed in a damping liquid. Like most of the direction indicating instruments on a plane its indications are only of significance when the plane is pursuing a steady course. On turns or rapid changes of direction of any sort perturbations prevent accurate reading.

The altimeter is of the common aneroid barometer type. On American instruments it is usually graduated to read in 100-foot steps. While somewhat sluggish, it is quite satisfactory for all ordinary determinations of altitude in photographic work. Were primary map making to be undertaken, where the scale was determinable only from the altitude and focal length of the lens, the ordinary altimeter is hardly accurate enough.

Fig. 11.—Appearance of the earth from a high altitude—10,000 feet or more.

The air speed indicator consists of a combination of Venturi and Pitot tubes, producing a difference of pressure when in motion through the air which is measured on a scale calibrated in air speed. This instrument is important for determining, in combination with wind speed, the ground speed of the plane, on the basis of which is calculated the interval between exposures to secure overlapping photographs. Its accuracy is well above that necessary for the purpose.

Inclinometers for showing the lateral and fore and aft angle of the plane with the horizontal, are occasionally used, and have also been incorporated in cameras. The important point to remember about these instruments is that they are controlled not alone by gravity but as well by the acceleration of the plane in any direction. They consequently indicate correctly only when the plane is flying straight. On a bank the lateral indicator continues to indicate “vertical” if the bank is properly calculated for the turn.

II
THE AIRPLANE CAMERA

CHAPTER III
THE CAMERA—GENERAL CONSIDERATIONS

Chief Uses of an Airplane Camera.—The kinds of camera suitable for airplane use and the manner in which they must differ from cameras for use on the ground are determined by consideration of the nature of the work they must do. Four kinds of pictures constitute the ordinary demands upon the aerial photographer. These are single objectives or pin points, mosaic maps of strips of territory or large areas, oblique views, and stereoscopic views. Each of these presents its own peculiar problems influencing camera design.

Pinpoints consist of such objects as gun emplacements, railway stations, ammunition dumps, and other objects of which photographs of considerable magnification are desired for study. Here the instrumental requirements are sufficient focal length of lens to secure an image of adequate size; means for pointing the camera accurately; enough shutter speed to counterbalance the speed of the plane; sufficiently wide lens aperture to give adequate exposure with the shutter speed required; means of supporting the camera to protect it from the vibration of the plane.

Mosaic maps are built up from a large number of photographs of adjacent areas. In addition to the above requirements, mosaic maps demand lenses free from distortion and covering as large a plate as possible, in order to keep to a minimum the number of pictures needed to cover a given area; means for keeping the camera accurately vertical, and means for changing the plates or films and resetting the shutter rapidly enough to avoid gaps between successive pictures. At low altitudes and high ground speeds the interval between exposures becomes a matter of only a few seconds.

Oblique views are made at angles of from 12 to 35 degrees from the horizontal, usually from comparatively low altitudes. They have been found to be particularly suitable for the use of men who have no training in photographic interpretation, being more like the pictures with which the men are familiar. Distributed among the infantry before an attack, they have proved indispensable aids to the proper knowledge of the ground to be covered. The additional requirement here is for high shutter speed to eliminate the effect of the relatively very rapid movement of the foreground.

Stereoscopic views are among the most useful of all airplane pictures. They are made from successive exposures, the separation of the points of view being obtained not by two lenses at the distance of the eyes apart, but by the motion of the plane. For this purpose the views should overlap by at least 60 per cent; this, therefore, requires a very short interval between exposures. For stereo-oblique views this may mean that they are taken at intervals as short as one or two seconds.

Chief Differences between Ground and Air Cameras.—Certain definite differences are thus seen to stand out between airplane cameras and the ordinary kind. It is essential that the apparatus for use in the air shall have high lens and shutter speed, means for rapid changing of plates, and anti-vibration suspension. Without these features a camera is of little use for aerial work. These requirements lead inevitably to greater complexity of design. One simplification over ground cameras, however, is brought about by the fact that all exposures are made on objects beyond the practical infinity point of the lens; consequently, all cameras are fixed focus. This fixed focus feature is a positive advantage in construction, since it permits of the simple rigid box form, desirable and necessary to withstand the strains due to the weight of the lens and the stresses from the plane. But with the abandonment of all provision for focussing in the air must go special care that the material used in constructing the camera body is as little subject as possible to expansion and contraction with temperature, since there is often a drop of 30 to 40 degrees Centigrade from ground to upper air. The effect of change of temperature on focus will be treated in the discussion of lenses.

In addition to these differences, we must keep in mind certain requirements which are conditioned by the nature and place of aerial navigation. Thus all mechanical devices which will fail to function at the low temperatures and pressures met at high altitudes are entirely unsuitable. Experience has shown, too, that we must avoid all mechanism depending primarily on springs and on the action of gravity. Vibration, and the motion of the plane in all three dimensions, conspire to render mechanical motions unreliable when actuated by these agencies. All plate changing, shutter setting, and exposing operations should be as nearly as possible positively controlled motions. Because of the cold of the upper air all knobs, levers and catches must be made extra large and easy to handle with heavy gloves. Circular knurled heads to such parts as shutter setting movements are to be avoided in favor of bat-wing keys or levers. Grooves for the reception of magazines must be as large and smooth as possible, and guides to facilitate the magazines' introduction should be provided (Fig. [50]). No releases or adjustments which depend upon hearing or upon a delicate sense of touch are feasible in airplane apparatus. Wherever possible, large visible indicators of the stage of the cycle of operations should be provided. Loose parts are to be shunned, as they are invariably lost in service. Complete operating instructions should be placed on the apparatus wherever possible, to minimize the confusion due to changing and uninstructed personnel.

The Elements of the Airplane Camera.—Disregarding its means of suspension, the airplane camera proper consists essentially of lens, camera body, shutter, and plate or film holding and changing box.

In certain of the aerial cameras developed early in the war all of these elements were built together in a common enclosure. Later it was generally recognized that a unit system of interchangeable parts is preferable. In the case of the lens there arose various requirements for focal length, from 25 to 120 centimeters, according to the work to be done. Rather than use an entirely different camera for each different kind of work, it is better to have lenses of various focal lengths, mounted in tubes or cones, all built to attach to the same camera body. In the case of the shutter it is desirable to be able to repair or calibrate periodically. By making the shutter a removable unit, the provision of a few spares does away with the need for putting the whole camera out of commission. Similar considerations hold with reference to other parts.

A further material advantage that comes from making airplane cameras in sections is the greater ease with which they are inserted in the plane, usually through the openings between diagonal cross-wires. It is in fact only by virtue of this possibility of breaking up into small elements that some of the larger cameras could be inserted in the common types of reconnaissance plane. Illustrations of the building up of cameras from separate removable elements are given in the detailed discussion of the individual types.

Types of Airplane Cameras.—During the course of the war airplane cameras have been classified on various bases, in different services. In the French service, where the deMaria type of camera was standardized early in the war, the usual classification was based on focal length; thus the standard cameras were spoken of as the 26, the 50 and the 120 (centimeter). A further distinction was then made according to the size of plate, this being originally 13 × 18 centimeters for the 26 centimeter, and 18 × 24 centimeters for the larger cameras. In the English service the 4 × 5 inch plate was used almost exclusively, and their various types of cameras were known by serial letters—C, E, L, etc. Both these modes of classification became inadequate with the ultimate agreement to standardize on the 18 × 24 centimeter size for all plates, and to carry lenses of all focal lengths in interchangeable elements.

For purposes of description and discussion, it is most convenient to classify cameras according to their method of operation and the sensitive material employed. On this basis we may distinguish among cameras using plates three kinds—non-automatic cameras, semi-automatic cameras, and automatic cameras. We may similarly discuss film cameras, but having treated the plate cameras comprehensively, it will be found that the discussion of all types of film camera can be handled most conveniently by studying the differences in construction and operation introduced by the characteristics of film as compared to plates.

CHAPTER IV
LENSES FOR AERIAL PHOTOGRAPHY

General Considerations.—The design and selection of lenses for aerial photography present on the whole no problems not already encountered in photography of the more familiar sort. Indeed, the lens problem in the airplane camera is in some particulars more simple than in the ground camera. For instance, there is no demand for depth of focus—all objects photographed are well beyond the usually assumed “infinity focus” of 2000 times the lens diameter. Such strictly scientific problems of design as pertain to aerial photographic lenses are ones of degree rather than of kind. Larger aperture, greater covering power, smaller distortion, more exquisite definition—these always will be in demand, and each progressive improvement will be reflected in advances in the art of aerial photography. But many lens designs perfected before the war were admirably suited, without any change at all, for aerial cameras.

Of the utmost seriousness, however, with the Allies, was the problem of securing lenses of the desired types in sufficient numbers. The manufacture of the many varieties of optical glass essential to modern photographic lenses was almost exclusively a German industry, which had to be learned and inaugurated in Allied countries since 1914. In consequence of this entirely practical problem of quantity production without the glasses for which lens formulæ were at hand, some new lens designs were produced. Whether any of these possess merits which will lead them to be preferred over pre-war designs, when the latter can again be manufactured, remains to be seen.

While the glass problem was still unsolved, aerial cameras had to be equipped with whatever lenses could be secured by requisition from pre-war importation and manufacture, and later, with lenses designed to utilize those glasses whose manufacture had been mastered in the allied countries. It is important that the historical aspect of this matter be well understood by the student of aerial photographic methods, for the use of these odd-lot lenses reacted on the whole design of aerial cameras and on the methods of aerial photography, particularly in England and the United States. Almost without exception the available lenses were of short focus, considered from the aerial photographic standpoint; that is, they lay between eight and twelve inches. This set a limit to the size of the airplane camera, quite irrespective of the demands made by the nature of the photographic problem. Lenses of these focal lengths produced images which, for the usual heights of flying, were generally considered too small, and which were, therefore, almost always subsequently enlarged. Such was the English practice, which was followed in the training of aerial photographers in America, where exactly similar conditions held at the start with respect to available lenses. French glass and lens manufacturers did succeed in supplying lenses of longer focus (50 centimeters), in numbers sufficient for their own service, although never with any certainty for their allies. The French, therefore, almost from the start, built their cameras with lenses of long focus, and made contact prints from their negatives.

Practices adopted under pressure of an emergency to meet temporary practical limitations often come to dominate the whole situation. This is particularly true of aerial photography in the British and American services. The small apparatus built around the stop-gap short focus lenses fixed the plane designer's idea of an airplane camera, and the space it should occupy. This was directly reflected in the designs of the English planes, and the American planes copied after them. Meanwhile the American photographic service in France associated itself with the French service, adopting its methods and apparatus, and using French planes whose designs were not being followed in American construction. The task of harmonizing the photographic practice as taught in America, following English lines, with French practice as followed in the theater of war, and of adapting planes built on English designs so that they could carry French apparatus, was a formidable one, not likely to be soon forgotten by any who had a part in it.

Photographic Lens Characteristics.—Whole volumes have been written on the photographic lens, and on the optical science utilized and indeed brought into being by its problems. Such works should be consulted by those who intend to make a serious study of the design of lenses for aerial use. No more can be attempted, no more indeed is relevant here, than an outline review of the chief characteristics and errors of photographic lenses, considering them with special reference to aerial needs.

The modern photographic lens is, broadly speaking, a development of the simple convex or converging lens. Its function is the same: to form a real image of objects placed before it. But the difference in performance between the simple lens and the modern photographic objective is enormous. The simple lens forms a clear image only close to its axis, for light of a single color, and as long as its aperture is kept quite small as compared to the distance at which the image is formed. The photographic lens, on the other hand, is called upon to produce a clear image with light of a wide range of spectral composition, sharply defined over a flat surface of large area, and it must do this with an aperture that is large in comparison with the focal length, whereby the amount of light falling on the image surface shall be a maximum. This ideal is approximated to a really extraordinary degree by the scientific combination and arrangement of lens elements made from special kinds of glass in the best photographic lenses of the anastigmat type. The result is of necessity a set of compromises, whereby the outstanding errors are reduced to a size judged permissible in view of the work the lens is to do. These errors or aberrations are briefly reviewed below, in order that the reader may readily grasp the terms in which the performance and tolerances in aerial lenses are described.

Fig. 12.—Diagrammatic representation of spherical aberration.

Spherical Aberration and Coma.—Suppose we focus on a screen, by means of a simple convex lens the image of a distant point of light. Suppose for simplicity that this image is located on the axis of the lens and that light of only one color is used, such as yellow. It will be found that the smallest image that can be obtained is not a point, but a small disc. This is due to the fact that the rays of light passing through the outer portions of the lens are bent more than those passing through the lens in the region near the center. This effect is shown in Fig. [12] by the usual mode of representing it graphically. Here the figures 1, 2, 3, 4, represent distances from the axis of the lens, and the letters A₁, A₂, A₃, A₄, the points of convergence of the rays from 1, 2, 3, 4, etc. These distances projected upward on to the produced lens points form a curve which shows at a glance the extent and direction of the error due to each part of the lens. This information is of value where the lens is fitted with an adjustable diafram. With some types of correction sharper definition may be obtained by reducing the aperture. With others, however, diaframing impairs definition, by destroying the balance between under and over correction which averages to make a good image. In aerial lenses it is not customary to use diaframs, as all the light possible is desired. Consequently the reduction of spherical aberration must be accomplished by proper choice of lens elements and their arrangement.

Off the axis of the lens the image of a point source takes on an irregular shape, due to oblique spherical aberration or coma.

Chromatic Aberration.—Because of the inherent properties of the glass of which it is made, a simple collective lens does not behave in the same way with respect to light of different colors. If one attempts, with such a lens, to focus upon a screen the image of a distant white light, it will be found that the blue rays will not focus at the same point as the red rays, but will come together nearer the lens. Modern photographic lenses are compounded of two or more kinds of glass in such a way as to largely eliminate this defect, the presence of which is detrimental to good definition. Such lenses are called achromatic, and the property of a lens by virtue of which this defect is eliminated is called its chromatic correction.

Chromatic correction is never perfect, but two colors of the spectrum can be brought to a focus in the same plane, and to a certain extent the departure of other colors from this plane can be controlled. Off the axis of the lens outstanding chromatic aberration results in a difference in the size of images of different colors, known as lateral chromatism.

Like spherical aberration, chromatic aberration is a contributing factor to the size of the image of a point source, which determines the defining power of a lens. It is, however, an error whose effect is to some extent dependent on the kind of sensitive plate used. Two lenses may give images of the same size (in so far as it is governed by chromatic aberration), if a plate of narrow spectral sensitiveness is used, while giving images of different size on panchromatic plates of more extended color sensibility. The choice of the region of the spectrum for which chromatic correction is to be made is thus governed by the color of the photographically effective light. While in ordinary photography the blue of the spectrum is most important, in aerial work where color filters are habitually used with isochromatic plates the green is most important, and color correction centered about this region constitutes a real difference of design peculiar to aerial lenses. Similarly the general use of deep orange or red filters with red sensitive plates, for heavy mist penetration, would call for a shift of correction to that part of the spectrum.

Astigmatism and Covering Power.—Suppose the lens forms at some point off its axis an image of a cross. Suppose one of the elements of the cross to be on a radius from the center of the field, the other element parallel to a tangent. The rays forming the images of these two elements of the cross are subject to somewhat different treatment in their passage through the lens. The curvature of the lens surfaces is on the whole greater with respect to the rays from the radial element than to those from the tangential element. They are therefore refracted more strongly and come to a focus nearer the lens. The arms of the cross are consequently not all in focus at once. This error, termed astigmatism, is rather well shown in Fig. [15], where the images of the outlying concentric circles are sharp in the radial, but blurred in the tangential direction.

Astigmatism can be largely compensated for, and its character controlled. The most usual correction brings the two images in focus together both at the axis, and on a circle at some distance out. This second locus of coincidence may or may not be in the same plane as the first, depending on which disposition produces the best average correction. The mean between the two foci determines the focal plane of the lens, which is in general somewhat curved. The covering power of a lens is given by the size of the field which is sufficiently flat and free from astigmatism for the purpose for which the lens is used. This is largely determined by the astigmatism, but the other aberrations are also important.

Illumination.—The amount of light concentrated by the lens on each elementary area of the image determines its brightness or illumination. The ideal image would, of course, be equally bright over its whole area of good definition, and for lenses of narrow angle this is approximately true. But when it is desired to cover a wide angle the question of illumination becomes serious. The relationship between angle from the axis and illumination is that illumination is proportional to the fourth power of the cosine of the angle. This relationship is shown in the following table:

If the field of view is 60°, which corresponds to an 18 × 24 centimeter plate with a lens of 25 centimeter focus, the brightness is only 56 per cent., and the necessary exposure at the edge approximately 1.8 times that at the center. This effect is shown in Fig. [15]. It is very noticeable if the exposure is so short as to place the outlying areas in the under-exposure period.

Fig. 13.—Barrel and pin-cushion distortion.

Distortion.—Sometimes a lens is relatively free from all the aberrations, mentioned above, so that it gives sharp, clear images on the plate, yet these images may not be exactly similar to the objects themselves as regards their geometrical proportions; in other words, the image will show distortion. Lens distortion assumes two typical forms, illustrated in Fig. [13], which shows the result of photographing a square net-work with lenses suffering in the one case from “barrel” distortion and in the other from “pin-cushion” distortion. In the first the corners are drawn in relative to the sides; in the latter case the sides are drawn in with respect to the corners. Either sort is a serious matter in precision photography, such as aerial photographic mapping aspires to become. It must be reduced to a minimum and its amount must be accurately known if negatives are to be measured for the precise location of photographed objects. In general symmetrical lenses give less distortion than the unsymmetrical (Fig. [14]).

Fig. 14.—Arrangement of elements in two lenses suitable for aerial work: a, Zeiss Tessar; two simple and one cemented components (unsymmetrical); b, Hawkeye Aerial; two positive elements of heavy barium crown, two negative of barium flint, uncemented (symmetrical).

Lens Testing and Tolerances for Aerial Work.—Simple and rapid comparative tests of lenses may be made by photographing a test chart, consisting of a large flat surface on which are drawn various combinations of geometrical figures—lines, squares, circles, etc.—calculated to show up any failures of defining power. For testing aerial lenses the chart should be as large as possible, so that it may be photographed at a distance great enough for the performance of the lens to be truly representative of its behavior on an object at infinite distance. This means in practice a chart of 4 or 5 meters side, to be photographed at a distance 20 to 30 times the focal length of the lens.

Fig. 15.—Photograph of a lens testing chart, showing failure in defining power outside area for which the lens is calculated.

A typical photograph of such a chart is shown in Fig. [15]. It reveals at a glance the more conspicuous lens errors. At the sides and corners the concentric circles show the lens's astigmatism, by the clear definition of the lines radial to the center of the field and their blurring in the tangential direction. The falling off in illumination with increasing distance from the center is also exhibited; and the blurring of all detail outside the rectangle for which the lens was calculated shows that spherical, chromatic, and other aberrations have become prohibitively large.

But the only complete test of a lens is the quantitative measurement of errors made on an optical bench. A point source of light, which may at will be made of any color of the spectrum, is used as the object and its image formed by the lens in a position where it can be accurately measured for location, size, and shape by a microscope. A chart giving the results of such a test is shown in Fig. [16]. In the upper left-hand corner is shown the position of the focus for the different colors of the spectrum. Below this is recorded the lateral chromatism at 21 degrees, in terms of the difference in focus for a red and a blue ray. Below this again comes the distortion, or shift of the image from its proper position, for various angles (plotted at the extreme right) from the lens axis. To the right of this is the image size, at each angle, and finally, to the right of the diagram, are plotted the distances of the two astigmatic foci from the focal plane, together with the mean of the two foci, which practically determines the shape of the field.

An important point to notice is that these data are uniformly plotted in terms of a lens of 100 millimeters focal length irrespective of the actual focal length of the lens measured. Thus this particular chart is for a 50 centimeter lens but would be plotted on the same scale for a 25 or a 100 centimeter lens. Underlying this practice is the assumption that all the characteristics of lenses of the same design and aperture are directly proportional to their focal length. If this were so, then a 50 centimeter lens would give double the size of image that a 25 centimeter does, and so on. As a matter of fact, test shows that the size of the image does not increase so rapidly as the focal length; so that while the image size for a 25 centimeter lens would be, say, .05 millimeters per 100 millimeters focal length, it will be only .03 or .04 millimeters per 100 millimeters focal length for a 50 centimeter lens. The actual size of a point image will therefore be greater, though not proportionately greater.

Fig. 16.—Chart recording measurements of lens characteristics.

The chart presents tests on a good quality lens, and so gives a good idea of the permissible magnitude of the various errors. In many ways the most important figure is that for image size, including as it does the result of all the aberrations. In the example given, this varies from .075 to .15 mm. actual size. For the same type of lens of 25 centimeters focus this range will be from .05 to .10 mm. Since these are commonly used focal lengths, a good average figure for image size, commonly used in aerial photographic calculations, is ⅒ mm. In regard to astigmatic tolerances, the two astigmatic foci should not be separated at any point by more than 6 to 7 millimeters, and the mean of these should not deviate from the true flat field by more than ½ millimeter, in each case the figures being based on the conventional 100 millimeters focal length. Distortion should not be over .08 millimeter at 18° or .20 millimeter at 24° from the axis (per 100 millimeters focal length).

Lens Aperture.—In the simple lens the aperture is merely the diameter. In compound lenses the aperture is not the linear opening but the effective opening of an internal diafram. Photographically, however, aperture has come to have a more extensive meaning. While in the telescope the actual diameter of an objective is perhaps the most important figure, and in the microscope the focal length, in photography the really important feature is the amount of light or illumination. This is determined by lens opening and focal length together; specifically, by the ratio of the lens area to the focal length. The common system of representing photographic lens aperture is by the ratio of focal length to lens diameter, the lens being assumed to be circular. Thus F/5 (often written F.5) indicates that the diameter is one-fifth the focal length.

Two points are to be constantly borne in mind in connection with this system of representation. First, all lenses of the same aperture (as so represented) give the same illumination of the plate (except for differences due to loss of light by absorption and reflection in the lens system). This follows simply from the fact that the illumination of the plate is directly proportional to the square of the lens diameter, and inversely as the square of the focal length. Secondly, the illumination of the plate is inversely as the square of the numerical part of the expression for aperture. That is, lenses of aperture F/4.5 and F/6 give images of relative brightness (6
4.5)² = 1.78.

What lens aperture, and therefore what image brightness, is feasible, is determined chiefly by the angular field that must be covered with any given excellence of definition. The largest aperture ordinarily used for work requiring good definition and flat field free from distortion is F/4.5. Anastigmatic lenses of this aperture cover an angle of 16° to 18° from the axis satisfactorily, which corresponds to an 18 × 24 centimeter plate with a lens of 50 centimeters focus. Lenses with aperture as large as F/3.5 were used to some extent in German hand cameras of 25 centimeters focal length, with plates of 9 × 12 centimeters. English and American lenses of this latter focal length were commonly of aperture F/4.5, designed to cover a 4 × 5 inch plate.

As a general rule the greater the focal length the smaller the aperture—a relationship primarily due to the difficulty of securing optical glass in large pieces. Thus while 50 centimeter lenses of aperture F/4.5 are reasonably easy to manufacture, the practicable aperture for quantity production is F/6, and for 120 centimeter lenses, F/10. This means that a very great sacrifice of illumination must be faced to secure these greater focal lengths. As is to be expected from the state of the optical glass industry, the German lenses were of generally larger aperture for the same focal lengths than were those of the Allies. Besides the F/3.5 lenses already mentioned, their 50 centimeter lenses were commonly of aperture F/4.8, their 120 centimeter lenses of aperture F/7, or of about double the illuminating power of the French lenses of the same size.

Demands for large covering power also result in smaller aperture. The 26 centimeter lenses used on French hand cameras utilizing 13 × 18 centimeter plates were commonly of aperture F/6 or F/5.6. The lens of largest covering power decided on for use in the American service was of 12 inch focus, to be used with an 18 × 24 centimeter plate (extreme angle 26°); the largest satisfactory aperture for this lens is F/5.6.

Ordinarily the question of aperture is closely connected with that of diaframs, whereby the lens aperture may be reduced at will. Diaframs have been very little used in aerial photography. All the aperture that can be obtained and more is needed to secure adequate photographic action with the short exposures required under the conditions of rapid motion and vibration peculiar to the airplane. Any excess of light, over the minimum necessary to secure proper photographic action, is far better offset by increase of shutter speed or by introduction of a color filter. For this reason American aerial lenses were made without diaframs. In the German cameras, however, adjustable diaframs are provided (Fig. [43]), controlled from the top of the camera by a rack and pinion. In the camera most used in the Italian service an adjustable diafram is provided, but this is occasioned by the employment of a between-the-lens shutter of fixed speed, so that the only way exposure can be regulated is by aperture variation, a method which has little to recommend it.

The Question of Focal Length.—In aerial photography the lens is invariably used at fixed, infinity, focus. Under these conditions the simple relationship holds that the size of the image is directly proportional to the focal length and inversely proportional to the altitude. If any chosen scale is desired for the picture the choice of focal length is determined by the height at which it is necessary to fly. This at least would be the case were there no limitation to the practicable focal length—which means camera size—and were one limited to the original size of the picture as taken; that is, were the process of enlargement not available. But the possibility of using the enlarging process brings in other questions: Is the defining power of a short focus lens as good in proportion to its focal length as that of a long focus lens? If so a perfect enlargement from a negative made by a short focus lens would be identical with a contact print from a negative made with a lens of longer focus. Is defining power lost in the enlarging process with its necessary employment of a lens which has its own errors of definition and which must be accurately focussed?

Certain factors which enter into comparisons of this sort in other lines of work, such as astronomical photography, play little part here. These are, first, the optical resolving power of the lens, which is conditioned by the phenomena of diffraction, and is directly as the diameter; and, second, the size of the grain of the plate emulsion. The first of these does not enter directly, because the size of a point image on the axis of the lens, due merely to diffraction, is very much less than that given by any photographic lens which has been calculated to give definition over a large field, instead of the minute field of the telescope. Yet it may contribute toward somewhat better definition with a long focus lens because of the actually larger diameter of such lenses. The second factor is not important, because, as will be seen later, the resolving power of the plates suitable for aerial photography is considerably greater than that of the lens. The emulsion grain is in fact only a quarter or a fifth the size of the image as given by a 25 centimeter lens, and enlargements of more than two or three times are rarely wanted.

A series of experiments was made for the U. S. Air Service to test out these questions, using a number of representative lenses of all focal lengths, both at their working apertures and at identical apertures for all. With regard to lens defining power, as shown by the size of a point image, the answer has already been reported in a previous section. Lenses of long focus give a relatively smaller image than lenses of the same design of short focus. In regard to the whole process of making a small negative and enlarging it, the loss of definition is quite marked, as compared to the pictures of the same scale made by contact printing from negatives taken with longer focus lenses.

This answer is clear-cut only for lenses calculated to give the same angular field. Thus a 10 inch lens covering a 4 × 5 inch plate has about the same angle as a 50 centimeter lens for an 18 × 24 centimeter plate. When, however, it comes to the longer foci, such as 120 centimeters, the practical limitation to plate size (18 × 24 cm.) has been passed, and the angular field is less than half that of the 50 centimeter lens. The 120 centimeter lens need only be designed for this small angle, with consequent greater opportunities for reduction of spherical aberration. It is therefore an open question whether a 50 centimeter lens designed to cover a plate of linear dimensions 50
120 times that used with the regular 50 centimeter lens could not be produced of such quality that it would yield enlargements equal to contacts from a 120 centimeter lens. If so, lenses of larger aperture could be used, and a considerable saving in space requirements effected.

Focal lengths during the Great War were decided by the nature of the military detail which was to be revealed and by the altitudes to which flying was restricted in military operations. In the first three years of the war the development of defences against aircraft forced planes to mount steadily higher, so that the original three or four thousand feet were pushed to 15,000, 18,000, and even higher. Lenses of long focus were in demand, leading ultimately to the use of some of as much as 120 centimeters (Fig. [41]). In the last months of the war the resumption of open fighting made minute recording of trench details of less weight, while the preponderance of allied air strength permitted lower flying. In consequence, lenses of shorter focus and wider angle came to the fore, suitable for quick reconnaissance of the main features of new country. At the close of the war the following focal lengths were standard in the U. S. Air Service, and may be considered as well-suited for military needs. Peace may develop quite different requirements.

The question of the use of telephoto lenses in place of lenses of long focus is frequently raised. Lenses of this type combine a diverging (concave) element with the normal converging system, whereby the effect of a long focus is secured without an equivalent lens-to-plate distance. This reduction in “back focus” may be from a quarter to a half. Were it possible to obtain the same definition with telephoto lenses as with lenses of the same equivalent focus, they would indeed be eminently suitable for aerial work because of their economy of length. But experience thus far has shown that the performance of telephoto lenses, as to definition and freedom from distortion, is distinctly inferior, so that it is best to hold to the long focus lens of the ordinary type.

Lenses Suitable for Aerial Photography.—Among the very large number of modern anastigmat lenses many were found suitable for airplane cameras and were used extensively in the war. A partial list follows: The Cooke Aviar, The Carl Zeiss Tessar, the Goerz Dogmar, the Hawkeye Aerial, the Bausch and Lomb Series Ic and IIb Tessars, the Aldis Triplet, the Berthiot Olor.

The Question of Plate Size and Shape.—Plate size is determined by a number of considerations, scientific and practical. If the type of lens is fixed by requirements as to definition, then the dimensions of the plate are limited by the covering power. From the standpoint of economy of flights and of ease of recognizing the locality represented in a negative, by its inclusion of known points, lenses of as wide angle as possible should be used. If the focus is long, this means large plates, which are bulky and heavy. If the finest rendering of detail is not required a smaller scale may be employed, utilizing short focus lenses and correspondingly smaller plates. Thus a six inch focus lens on a 4 × 5 inch plate would be as good from the standpoint of angular field as a 12 inch on an 8 × 10 inch plate. This is apt to be the condition with respect to most peace-time aerial photography, which may be expected to free itself quickly from the huge plates and cameras of war origin.

For work in which great freedom from distortion of any sort is imperative, small plates will be necessary, for two reasons. One is that the characteristic lens distortions are largely confined to the outlying portions of the field. The other is that a wide angle of view inevitably means that all objects of any elevation at the edge of the picture are shown partly in face as well as in plan, which prevents satisfactory joining of successive views (Fig. [128]). In making a mosaic map of a city, if a wide angle lens is employed with large plates, the buildings lying along the junctions of the prints can be matched up only for one level. If this is the ground level, as it would be to keep the scale of the map correct, the roofs will have to be sacrificed. In extreme cases a house at the edge of a junction may even show merely as a front and rear, with no roof, while in any case the abrupt change at these edges from seeing one side of all objects to seeing the opposite side is not pleasing.

The table in a preceding section gives the relation of plate size to focal length found best on the whole for military needs. Deviations from these proportions in both directions are met with. In the English service the LB camera, which uses 4 × 5 inch plates, is equipped with lenses of various focal lengths, up to 20 inches. The German practice, as well as the Italian, was almost uniform use of 13 × 18 centimeter plates for all focal lengths. Toward the end of the war, however, some German cameras of 50 centimeter focal length were in use employing plates 24 × 30 centimeters.

It will be recognized that these plate sizes are chosen from those in common use before the war. A similar observation holds with even greater force on the question of plate shape. Current plate shapes have been chosen chiefly with reference to securing pleasing or artistic effects with the common types of pictures taken on the ground. These shapes are not necessarily the best for aerial photography. Indeed the whole question of plate shape should be taken up from the beginning, with direct reference to the problems of aerial photography and photographic apparatus.

A few illustrations will make this clear, taking Fig. [17] as a basis. If it is desired to do spotting (the photography of single objectives), the best plate shape would be circular, for that shape utilizes the entire covering area of the lens. If it is desired to make successive overlapping pictures, either for mapping, or for the production of stereoscopic pairs, a rectangular shape is indicated. If the process of plate changing is difficult or slow, it is advisable, in order to give maximum time for this operation, to have the long side of the rectangle parallel to the line of flight (indicated by the arrow). If economy of flights is a consideration, as in making a mosaic map of a large area, it is advantageous to have as wide a plate as the covering power of the lens will permit. Reference to Fig. [17] shows that this means a plate of small dimensions in the direction of flight. If the changing of plates or film is quick and easy, the maximum use of the lens's covering power is made by such a rectangle whose long side approximates the dimensions of the lens field diameter. This is in fact the choice made in the German film mapping camera (Figs. [61] and [63]), whose picture is 6 × 24 centimeters. An objection to this from the pictorial side, lies in the many junction lines cutting up the mosaic. Another objection, if the plane does not hold a steady course, is the failure to make overlaps on a turn. (Fig. [62].) Here as everywhere the problem is to decide on the most practical compromise between all requirements.

Fig. 17.—Possible choices of plate shape.

Focussing.—The process of focussing aerial cameras was at first deemed a mystery, though undeservedly so. A belief was long current that “ground” focus and “air” focus differ. In other words, that a camera focussed upon a distant object on the ground would not be in focus for an object the same distance below the camera when in the plane. Belief in this mysterious difference went so far that certain instruction books describe in detail the process of focussing a camera by trial exposures from the air.

Careful laboratory tests performed for the U. S. Air Service showed that neither low temperature nor low pressure, such as would be met at high altitudes, alter the focus of any ordinary lens by a significant amount, and that the possible contraction of the camera body was of negligible effect on the focus (not more than 1
200 per cent. per degree centigrade with a metal camera). In complete harmony with these tests has been the experience that if the ground focussing is done carefully, by accurate means, then the air focus is correct. The whole matter thus becomes one of precision focussing.

The best method, applicable if the air is steady, is to focus by parallax. The ground glass focussing screen is marked in the center with a pencilled cross. Over this is mounted, with Canada balsam, a thin microscope cover-glass. The camera is directed on an object a mile or more away, and the image formed by the lens is examined by a magnifying glass through the virtual hole formed by the affixed cover-glass. With the pencil line in focus the head is moved from side to side. If the image and pencil mark coincide they will move together as the head is moved. If the image moves away from the pencil mark and in the same direction as the eye moves, the image is too near the lens. If the image moves away in the opposite direction to the motion of the eye, it is too far from the lens. In either case the focus is to be corrected accordingly.

In place of a distant object, which may waver with the motion of the air, we may use an image placed at infinity by optical means. The collimator, an instrument for doing this, consists of a test object (lines, circles, etc.) placed accurately at the focus of a telescope objective. The camera lens is placed against this and focussed by parallax, as with a distant object. Collimators are employed in camera factories, and should be part of the equipment of base laboratories where repairing and overhauling of cameras is done.

Lens Mounts.—All that is required for the mounting of an aerial camera lens is a rigid platform, with provision for enough motion of the lens to adjust its focus accurately. As already explained, the lens works at fixed, infinity, focus, and therefore needs no adjustment during use. It must be held far more rigidly than would be possible by the bellows, which is an almost invariable adjunct of focussing cameras. The use of ordinary types of hand cameras on a plane is rarely successful just because of the bellows, which is strained and rattled by the rush of wind.

The lens mountings thus far used have been simple affairs. In the French cameras the lens is merely screwed into a flange which in turn is fastened by screws to a platform in the camera body. Adjustment for focussing is not provided; instead, the flange is raised on thin metal rings or washers, cut of such thickness by trial as to bring the lens to focus, once and for all.

The U. S. Air Service method of mounting is to provide the lens barrel with a long thread, which screws into a flange that in turn is mounted on a platform in the camera cone, by means of thumb-screws. The lens is focussed by screwing in and out, and then clamped by a screw through the side, bearing on the thread. The whole mount may be quickly removed by loosening the thumb-screws, and once focussed in one cone, can be transferred to another similar, machine-made cone without change of focus. Fig. [18] shows a 20 inch lens mounted in this manner. The photograph shows as well the ring on the front of the lens by means of which circular color filters may be held in place. This ring screws down on the filter, and the catch is dropped into the nearest vertical groove to the tight position.

Fig. 18.—50 centimeter F/6 lens in U. S. standard mount, showing color filter retaining ring and catch.

A somewhat different and better method of tightening the lens in the flange, when focussed, has been adopted in the English lens mount, which is in general similar to the American. The threaded part of the flange is split by a slot cut parallel to the flange base, and a screw is run into the flange from the front, through the split portion. By tightening this screw, which is always accessible, the split part of the flange is squeezed together, thus rigidly holding the lens barrel.

CHAPTER V
THE SHUTTER

Permissible Exposure in Airplane Photography.—A definite limitation to the length of exposure in airplane cameras is set by the motion of the plane. If we represent the speed of the plane by S, the altitude of the plane by A, and the focal length of the lens by F, we obtain at once from the diagram (Fig. [19]), that s, the rate of movement of the image on the plate, is given by the relation,

If we call the permissible movement d, then the permissible exposure time, t, is given by the relation—

As a representative numerical case, expressing all quantities in centimeters and in centimeters per second, let F = 50, S = 20,000,000
3600 (200 kilometers per hour), and A = 300,000, then

If we take for the permissible undetectable movement, .01 centimeter, which is, as has been shown, a reasonable figure for lens defining power, we have, then, that the longest permissible exposure is .011 second—in round numbers, one-hundredth.

In flying with a slow plane, or in flying against the wind, the exposure can sometimes be increased to as much as double this length. Diminishing F would similarly extend the allowable exposure, but the ratio of F to A approximates to a constant in actual practice; in other words, a certain resolution and size of image have been found desirable. If flying is forced higher, a longer focus lens is used; if lower flying is possible, a lens of shorter focus. This relationship has, of course, been derived from war-time experience. Probably much of the prospective peace-time mapping work will impose substantially easier requirements as to definition and will thus allow longer exposures.

Fig. 19.—Relative motion of plane and photographic image.

For low oblique views the longest exposure is much less. Taking 45 degrees as a representative angle for the foreground, and 500 meters as a representative height, the value of t becomes 1
600.

These figures will illustrate two important points: they show how severe is the limitation as to exposure, with the consequent heavy demand on lens and sensitive material speed; and they show how important it is to secure a shutter with the maximum light-giving power for a specified length of exposure. This leads to a study of the characteristics as to efficiency of the two common types of shutter, namely, shutters at or between the lens, and focal-plane shutters.

Characteristics of Shutters Located at the Lens.—Of the various shutters located at the lens the most common is the type that is clumsily but descriptively termed the “between-the-lens” shutter. This is composed of thin hard rubber or metal leaves or sectors which overlap and which are pulled open to make the exposure. It may require two operations, one for setting and one for exposing, or it may, as in some makes, set and expose by a single motion. Clock escapements, or some form of frictional resistance, are depended on to control the interval between opening and closing. This shutter is the one almost universally employed on small hand cameras and on all lenses up to about two inches diameter. It gives speeds sometimes marked as high as 1
300 second, although usually not over 1
100 on actual test.

Between-the-lens shutters have been used to some extent on the shorter focus (up to 25 centimeter) aerial cameras, notably in the Italian service. They suffer, however, from two limitations. In the first place we have not yet solved the mechanical problems met with in trying to make the shutter of large size (as for 50 centimeter F/6 lenses) at the same time to give high speeds. In the second place the efficiency of the type is low because a large part of the exposure time is occupied by the opening and closing of the sectors.

If we define the efficiency of a shutter as the ratio of the amount of light it transmits during the exposure to the amount of light it would transmit were it wide open during the whole period, then the efficiency of the ordinary between-the-lens shutter is of the order of 60 per cent. This means 1.6 times the motion of the image for the same photographic action that we should have with a perfect shutter. The accompanying photographic record (Fig. [20]) of the opening and closing process of this type of shutter clearly illustrates its deficiencies.

Fig. 20.—Effective lens opening at equal intervals of time: (a) during focal plane shutter exposure; (b) during between-the-lens shutter exposure.

Characteristics of the Focal-Plane Shutter.—Long before the days of aerial photography the problem of a high-efficiency high-speed shutter for photographing moving objects on the ground—railway trains or racing automobiles—had already led to the development of the focal-plane shutter. This is a type peculiarly adapted to the problems of the airplane camera. It consists essentially of a curtain, running at high speed close to the photographic plate, the exposure being given by a narrow rectangular slot.

If the focal-plane shutter is in virtual contact with the sensitive surface the efficiency, as defined above, is 100 per cent., since the whole cone of rays from the lens illuminates the plate during the whole time of exposure. But if the curtain is not carried close to the plate the efficiency falls off rapidly with distance, especially so for small apertures of the slot.

Fig. 21.—Calculation of focal plane shutter efficiency.

The efficiency of the focal-plane shutter may be calculated as follows: Let the focal length of the lens be F, its diameter be F
N, the width of the slot be a, and the distance from plate to curtain d (Fig. [21]). Now if the curtain is moving at a uniform speed, the time taken for the slot to traverse the whole cone of rays, from the instant it enters till the instant it leaves, will be directly proportional to

If the curtain were in contact with the plate the time taken for the same amount of light to reach the sensitive surface would be proportional to a. Again defining shutter efficiency as the ratio of the light transmitted to what would have been transmitted were the shutter fully open for the total time of exposure, the efficiency, E, is given at once by the expression—

As an example let the lens aperture be F/6, so that N = 6; let d = 1, and a = 1, then E = 6
7. In the French deMaria cameras, where d = 4 centimeters, E = 60 per cent. for the aperture assumed, which is representative. Fig. [22] exhibits diagrammatically the chief characteristics of the focal plane shutter.

Fig. 22.—Characteristics of focal plane shutter.

In view of the necessity for some distance between shutter and plate it is obviously important to keep a as large as possible, depending for the requisite shutter speed on the velocity of the curtain. Large aperture and high curtain speed are also found to be desirable when we consider the distortion produced by the focal-plane shutter.

Distortions Produced by the Focal-plane Shutter.—While the time of exposure of any point on the plate can, with the focal-plane shutter, easily be made 1
100 second or less, the whole period during which the shutter is moving is much greater than this. For instance, a 1 centimeter opening which gives 1
100 second exposure takes ⅒ second to move across a 10 centimeter plate, or nearly ⅕ second for an 18 centimeter plate. With a moving airplane this means that the point of view at the end of the exposure has moved forward compared to that at the beginning, by the amount of motion of the plane in the interval. If the shutter moves in the direction of motion of the plane the image will be magnified; if in the opposite direction, it will be compressed along the axis of motion. The amount of this distortion is calculated as follows:

Let the velocity of the plane be V, and that of the shutter be v. Let the focal length of the camera be F, and the altitude A. If the camera were stationary, a plate of length l would receive on its surface an image corresponding to a distance A
F × l on the ground. Due to the motion of the shutter the end of the exposure occurs at a time l
v after the start. In this time the plane has moved a distance V × l
v; hence the point photographed at the end of the shutter travel is Vl
v within or beyond the original space covered by the plate, depending on the direction of motion of the curtain. The distortion, D, is given by the ratio of this distance to the length corresponding to the normal stationary field of view:

When V = 200 kilometers per hour, v = 100 centimeters per second, F = 50 centimeters, A = 3000 meters, we have—

Or if the actual distance error on the ground is desired,

As a percentage error this one per cent. is small compared with other uncertainties, such as film shrinkage or the error of level of the camera. As an absolute error in surveying, thirty feet is, of course, excessive.

The distortion is diminished for any specified shutter speed by making the speed of travel of the curtain as large as possible and by correspondingly increasing the aperture. In connection with film cameras, another solution which has been suggested is to move the film continuously during the exposure in the direction of the plane's motion. The requisite speed of the film v' to eliminate distortion is given by the relation:

For the values of V, F, and A used above, v' = .92 centimeters per second. This speed is clearly that which holds the image stationary on the film—a fact which suggests another object for such movement, namely, to permit of longer exposures.

The effect of focal plane distortion may be averaged out in the making of strip maps, if the shutter is constructed so as to move in opposite directions on successive exposures. The first picture will be magnified, the second compressed, and so on, but a strip formed of accurately juxtaposed pictures will be substantially accurate in over-all length. Such a shutter is embodied in one of the German film cameras (Fig. [61]).

Distortion of the kind above discussed is absent with between-the-lens shutters, which may conceivably be improved in efficiency and in feasible size. If so they would merit serious consideration for aerial mapping.

Methods and Apparatus for Testing Shutter Performance.—With a focal-plane shutter the desirable qualities in performance are three in number: (1) Adequate speed range, which may be taken as from 1
50 to 1
500 second for aerial work, (2) good efficiency, which has already been treated, and (3) uniformity of speed during its travel across the plate. Before the advent of aerial photography little attention was paid to speed uniformity, differences of 50 per cent. in initial and final speed being common in focal-plane shutters, and but little noticed in ordinary landscape work because of the natural variation of brightness from sky to ground. In the making of aerial mosaic maps the non-uniformity of density across the plate results in a most offensive series of abrupt changes of tone at the junction points of the successive prints (Fig. [140]), an effect which must be minimized by manipulation of the printing light.

Instruments for testing the speed and uniformity of action of focal-plane shutters are an essential part of any laboratory for developing or testing photographic apparatus and some simple device for setting and checking shutter speed should be available in the field. Every such speed tester must contain some form of time counting element—pendulum, tuning fork or clock-work. Elaborate shutter testers, suitable for determining all the characteristics of all types of shutter, have been developed and used in certain of the photographic research laboratories. For the study and setting of focal-plane shutters (whose efficiency need not be measured, as it can be simply calculated from linear dimensions), the following simple kinds of apparatus are adequate:

Fig. 23.—Apparatus for testing focal plane shutter speed throughout the travel of the curtain.

Clock dial type of shutter tester. This consists essentially of a black clock dial carrying a white pointer which makes its complete revolution in one second or less. If this dial is photographed by the camera under test, the width of the sector traced during the exposure by the moving pointer shows the time interval. If the dial is photographed at several points on the plate—beginning, middle and end of the shutter travel—the complete characteristics of the shutter can be determined.

Interrupted light type of shutter tester. For the study of uniformity of shutter action alone the apparatus shown in Fig. [23] may be employed. A is a high intensity light source, such as an arc or a gas filled tungsten lamp. L is a convex lens, focussing an image of the light source on a small aperture in the screen E. D is a sector disc which, driven by the motor M, interrupts the transmitted light with a frequency determined by the number of openings of the sector and by the speed of rotation, which must be measured by a tachometer. The light diverging from the aperture in E falls upon the shutter S, which for this test is reduced to a narrow slit of one millimeter or less. Passing through the shutter opening the light falls upon the photographic plate P. The principle is simple: If the light is uninterrupted, the plate P is exposed at all points; due to the interruptions, a series of parallel lines of photographic action result, and their distance apart gives a measure of the speed of the shutter at any chosen point in its travel. A performance curve of the French Klopcic shutter is shown in Fig. [24]. The variation in speed lies over a range of two to one. So serious is this defect in these shutters that diaframs are sometimes inserted in the French cameras to cut off part of the light from the lens on the most exposed end of the plate. This expedient produces uniformity of photographic action, but does not overcome the movement of the image, which is one of the chief faults of excessive exposure.

Fig. 24.—Performance of Klopcic shutter.

Fig. 25.—Optical system of shutter tester for Air Service, U. S. Army.

A more complete apparatus, adapted both to absolute speed determinations and to the study of uniformity of action, is that worked out and used in the United States Air Service (Fig. [25]). At A is a high intensity light source, an image of which is focussed by the lens L₁ upon a slit E, in front of which stands a tuning fork T, of period 1024 or 2048 per second. The light diverging from the slit is received by a second lens, L₂ which is arranged either to focus the slit image upon the shutter curtain or to render the rays parallel, so that an entire camera may be inserted. In the latter case the camera lens L₃ serves to focus the slit image on the curtain C. After passing through the curtain aperture the light is focussed by the lens L₄ on the rotatable drum D, which carries a strip of sensitive film.

The operation of testing a shutter consists in focussing the slit image on the portion of the shutter whose performance is required, striking the tuning fork to set it vibrating, rotating the drum rapidly and setting off the shutter. There is thus obtained on the sensitive film an exposed strip resembling in appearance the edge of a saw, the number of teeth showing the time interval in vibrations of the tuning fork. Three exposures usually give all the points necessary for a practical knowledge of the shutter's uniformity of action. A point of some importance, learned from numerous shutter tests, is that a focal-plane shutter should be tested in the position in which it is to be used. Aerial camera shutters should be tested in the horizontal position.

Types of Focal-plane Shutters.—A variety of means have been utilized for securing the necessary variation in speed in focal-plane shutters. Their success is to be measured by the actual speed range and by the uniformity of speed attained. In aerial cameras at present in use we find variable tension of the curtain spring, the aperture being fixed; variable opening with fixed tension; multiple curtain openings with fixed spring tension; and combinations of two or all of these methods of speed control. The problem of covering the aperture during the operation of winding up or setting the shutter has led to further elaborations of shutter mechanism. These take the form of lens or shutter flaps, auxiliary curtains, and shutters of the self-capping type. Shutters embodying all these features are briefly described below.

Representative Shutters.—The Folmer variable tension shutter is used on the United States Air Service hand-held and hand-operated plate camera and on some of the film cameras. It consists of a fixed aperture curtain wound on a curtain roller in which the spring can be set to various tensions, numbered 1 to 10. The range of speeds attainable is at best about three to one, or from 1
100 to 1
300 second, considerably shorter than the range indicated as desirable. Its uniformity of travel is variable with the tension, as shown by representative performance curves in Fig. [30]. Lacking any self-capping feature the shutter is provided either with an auxiliary curtain, or in the hand-held camera with flaps in front of the lens, opened by the exposing lever before the curtain is released (Fig. [39]). This shutter is made a removable unit in the 18 × 24 centimeter hand-operated camera, but is built into the hand-held and film cameras.

Fig. 26.—Removable four-slit shutter of German (Ica) camera, showing flaps.

The Ica shutter used on the standard German aerial cameras is a good example of the multiple slit curtain (Fig. [26]). Four fixed aperture slits are provided, with a single tension, the openings roughly in the ratio 1, ½, ¼, ⅛, which when the spring tension is properly adjusted give exposures of 1
90, 1
180, 1
375, 1
750 second. To pass from one exposure time to another the setting milled head is wound up to successively higher steps or else exposed one or more times without resetting, depending on the direction it is desired to go. Capping during setting, or during exposure, in order to change the opening, is provided for by a pair of flaps on the shutter unit, which open into the camera body. The mechanical work on these shutters is of excellent quality, the curtain running with exceptional smoothness. Provision is made for adjusting the tension until the marked speeds are attained; this is presumably done in a repair laboratory to which the shutter only need be sent, as it is a removable unit. Tests made on one of these shutters wound to its highest tension are shown in Fig. [30]. The marked speeds are not attained, and there is considerable lack of uniformity from start to finish of the travel.

L camera variable-aperture shutter. The shutter of the L type camera (Fig. [27]) is representative of one of the most primitive methods of varying aperture. The two jaws of the slit are held together by a long cord passing completely around the aperture, fastened permanently at one end and attached at its other end by a sliding clasp or saddle. As this saddle is forced in one direction the slit is closed, in the opposite direction the cord becomes slack, and after the shutter is released once or twice the slit assumes a wider opening. A chronic trouble is the breaking of the cords. Its opening can be changed only after the plate magazine is removed.

Fig. 27.—“L” type camera showing open negative magazines and shutter mechanism.

U. S. Air Service variable-aperture shutter. This shutter is incorporated in the American deRam and in other late American cameras (Fig. [28]). Its characteristic feature is the introduction of an idler, whose distance from the main curtain roller can be varied. Tapes whereby the following curtain is attached to the spring roller pass over this idler, and by changing its position the aperture or distance between the two curtain elements is altered over a large range. Tests of this shutter are shown in Fig. [30]. A speed of 1
50 second is provided for by a slit width of five centimeters, and the highest speed is fixed only by the practical limit of approach of the jaws. Experiment shows great uniformity of rate of travel to be attainable by combining careful choice of spring length and tension with good workmanship in the mechanical features. Variable-aperture fixed-tension shutters have a definite advantage over the variable-tension type in that they can utilize for all speeds that tension which gives uniform action. The capping feature of this shutter is provided in the American deRam by flaps, in the automatic film camera by an auxiliary curtain. The shutter is removable in the deRam, but built into the other camera.

Fig. 28.—Variable aperture curtain developed in U. S. Air Service, and used in American deRam, and “K” type automatic film cameras.

The Klopcic variable-tension, variable-aperture, self-capping shutter is an example of an attempt to meet all shutter requirements with an entirely self-contained mechanism. It is shown diagrammatically in Fig. [29]. Tapes G₁, G₂ are used to connect the following curtain B directly to the spring roller T, at a fixed distance, while the leading curtain, A, may be slid along the tapes by small friction buckles, C₁, C₂, auxiliary springs R₁, R₂ serving to keep it taut in any position. When the shutter is being set the buckles are arrested against stops while the winding-up continues for what is to be the following half of the curtain in exposing. When released the curtain moves across with an aperture fixed by the point of setting of the buckle stops. At the end of the travel the buckles are arrested by other stops, while the following portion of the curtain continues its travel to the end. On re-winding, therefore, the aperture is closed. Variable tension as well as variable aperture is provided, although little used. In the French cameras a lens flap is also inserted behind the lens, but this is not needed if the self-capping feature functions properly. On the hand cameras this flap is said to be necessary in order to prevent a curious kind of accident: if the camera is held on the knee, pointing upward, an image of the sun may be formed on the curtain and burn a hole through it.

Fig. 29.—Mechanism of Klopcic variable aperture self-capping shutter.

The performance of the French shutter in respect to uniformity has already been shown in Fig. [24]. It leaves very much to be desired. Besides non-uniformity of action during its travel it exhibits another common defect of variable-tension shutters, namely, the curtain must be released several times after a change of tension before the new speed is established (Fig. [30], tensions 5 and 5´).

Fig. 30.—Performances of various shutters used on aerial cameras. Speeds expressed in reciprocals of fractional parts of one second.

The French shutter as made for the deMaria cameras is a removable unit. The small size (13 × 18 cm.) sets by the straight pull of a projecting pin, the larger (18 × 24 cm.) by winding up a milled head. The former is the more convenient motion for an aerial camera. Care must be taken with either type that the motion of setting is not stopped when the first resistance is encountered; this occurs when the tape buckles strike their stop and the slit begins to open.

CHAPTER VI
PLATE-HOLDERS AND MAGAZINES

In the earlier days of airplane photography the ordinary plate-holder or double dark slide was used to some extent, but it is ill-suited to the purpose because of the considerable time and attention required for its operation. It has nevertheless the merit of adding little to the length of the camera, and it works in any position. For these reasons it has remained in occasional use for the taking of oblique views with long focus cameras in a cramped fuselage.

Next in order of progress rank the simple box magazines, for holding a dozen, eighteen or twenty-four plates, as used in the English C, E, and L type cameras. These are little more than boxes with sliding lids which when open permit the introduction or removal of the plates. Figs. [45] and [46] illustrate the magazine of this type as made for the English C and E cameras. It is constructed of wood, grooved to fit tracks on the camera, and is furnished with a sliding door or lid hinged in the middle to fold down out of the way when open. The eighteen plates are carried in metal sheaths, both to provide opaque screens between them, and to protect them from injury in the mechanism of the camera. Fig. [27] shows the all-metal magazine made for the American model L camera. This differs from the English in material of construction, plate capacity (24 instead of 18) and manner of operating the slide, which is built up of three thicknesses of phosphor bronze and draws out through metal guides bent into semicircular form. A snap catch holds this slide at either end of its travel. The leather strap introduced in the American model for carrying and handling is a distinct improvement. These magazines contain no springs or other mechanism, as the cameras with which they are used depend upon the action of gravity for emptying the upper (feeding) magazine, and filling the lower (receiving) one.

Fig. 31.—Aerial hand camera (U. S. type A-2).

Next in order of complexity may be ranked the bag magazine (Figs. [31] and [44]). In this the exposed plate is pulled out of the magazine proper by a metal slide or rod into a leather bag. The rod is then pushed back, the plate in its metal sheath is grasped through the leather bag, lifted to the back of the magazine, and forced in behind the other plates. The number of plates exposed is indicated either by numbers on the backs of the sheaths, visible through a red glazed opening in the back, or else by a counter actuated by the metal slide rod. Usually twelve are carried in a magazine. For aerial work the common design of this magazine as used for ground work must be modified by providing extra large easily grasped hooks both on the draw rod and on the dark slide, which must be drawn before making the first exposure and replaced after the last. The small rings and grips of the standard commercial magazine are almost impossible to handle through heavy gloves.

The next type of magazine is represented by three designs, the Gaumont and deMaria, used very generally by the French during the war, and the Ernemann, used almost universally in the German air service (Figs. [32], [40] and [42]). In all of these the operation of plate changing is the same: the end of the magazine is pulled out and thrust back, a more simple operation than the bag manipulation just described. The internal workings are different according to size. In the smaller French magazines (13 × 18 cm.) the camera is first pointed upward, all the plates are drawn out except the one to be changed, and this, with the aid of springs, drops to the bottom, after which the other plates push back over it. The plates pull out in the direction of their long dimension. In the larger French magazine (18 × 24 cm.) only the exposed plate pulls out. The pull is in the direction of the shorter dimension of the plate, which is lifted up by heavy springs and slides back over the top of the pile. In the Ernemann magazine only six plates are carried, which there is good reason to believe represent the maximum feasible number, judging by the reports of jambs and breakages in the twelve-plate French magazines. In all of these magazines laminated wood slides pull out and in at each operation, and while satisfactory if made and operated in one climate, experience indicates that if made in America and sent abroad swelling of the wood may be expected to prevent their successful operation.

Fig. 32.—Various plate magazines used on aerial cameras.

Alternative forms of magazine, somewhat more practical from the standpoint of manufacture and export, are several designs embodying two compartments (Fig. [32]). In the most simple of these the plates are moved, immediately before or after exposing, from the unexposed to the exposed side. Illustrative of this type are the Folmer designs, in which the to-and-fro motion is imparted by a rack geared to a pinion actuated either by a lever, in the hand camera, or by the power drive, in the automatic design (Figs. [33] and [53]). Another illustration is afforded by the Piserini and Mondini magazine, in which the operation of changing is performed by a back-and-forth motion of a hand-grip, which also sets the camera shutter (Fig. [47]).

Fig. 33.—U. S. Air Service hand camera, with two-compartment magazine.

Fig. 34.—Film type hand camera.

In these magazines the center of gravity changes as the exposed plates are moved over, and only half the inside space is occupied with plates. These objections are overcome in the Chassel form, where both compartments are always full. Transfer of the bottom exposed plate from one compartment to the other is compensated for by the simultaneous shift of the top plate in the receiving compartment, to the feeding side. In a modification of this idea by Ruttan the exposing position is when the plates are half-way through the shifting process, whereby the magazine may be symmetrically mounted on the camera body.

Fig. 35.—Apparatus for straightening plate sheaths.

Other more complicated magazines have been designed, some of which are shown in the diagrammatic ensembles of Figs. [32] and [48]. In the Jacquelin, the main body of plates is raised while the bottom (exposed) plate is folded against the side. The main body of plates then drops back to place, the exposed plate is carried on upward and folds down on the back of the pile. The Bellieni magazine system uses three, a central feeding one and two below for receiving, one on each side of the camera body. These were made solely for attachment to captured German cameras. In the Fournieux magazine the plates are carried in an interior rotating box. The plate to be exposed is dropped off the front of the pile, down to the focal plane, and after exposure is picked up and placed at the back of the pile, which has turned over in the meanwhile. The deRam rotating magazine is described in connection with the camera of which it is an essential part (Fig. [52]).

Fig. 36.—Training plane equipped for photography, showing “L” camera in floor mount and magazine rack forward of the observer.

For the protection of the plates during their manipulation, and in the camera, all plate magazines thus far developed carry them in thin metal sheaths. These add greatly both to the weight and to the time necessary to handle the plates, but no means have as yet been found for dispensing with them. Fig. [35] shows a representative sheath or septum, as used in the L camera. On three sides the edge is bent up and turned over, forming a ledge for the plate to press against. The fourth side is left open for inserting the plate, which is then held in by a small upward projecting lip, and kept close against the ledges by narrow springs at the sides. To insert or remove the plate the projecting lip is depressed, either by springing the sheath by pressure from the sides or by using an appropriate tool.

Care of sheaths. Unless systematically taken care of, plate sheaths become bent or dented. They are then a menace to camera operation, catching or jamming in the plate changing process, breaking plates and damaging camera mechanisms. In order to maintain them flat and true, steel forms are necessary on which the sheaths may be hammered to shape with a mallet (Fig. [35]).

Magazine racks. Reconnaissance and mapping call for a number of exposures much greater than the capacity of one 12, 18, or 24 plate magazine. Additional magazines must therefore be carried. These should be in racks convenient to the observer (Fig. [36]), securely held yet capable of quick removal and insertion. In the rack designed to carry two of the metal magazines for the American L Camera, the magazines slide into loose grooves formed by a metal lip. They are prevented from slipping out by a spring catch, past which they slide when inserted but which is instantly thrown aside by pressure of the thumb as the hand grasps the magazine handle for removal.

CHAPTER VII
HAND-HELD CAMERAS FOR AERIAL WORK

Field of Use.—The first cameras to be used for aerial photography were hand-held ones of ordinary commercial types. Indeed the idea is still prevalent that to obtain aerial photographs the aviator merely leans over the side with the folding pocket camera of the department store show window and presses the button. But the Great War had not lasted long before the ordinary bellows focussing hand camera was replaced by the rigid-body fixed-focus form, equipped with handles or pistol grip for better holding in the high wind made by the plane's progress through the air. Even this phase of aerial photography was comparatively short-lived. The need for cameras of great focal length, and the need for apparatus demanding the minimum of the pilot's or observer's attention, both tended to relegate hand-held cameras to second place, so that they were comparatively little used in the later periods of the war.

Yet for certain purposes they have great value. They can be used in any plane for taking oblique views, and for taking verticals, in any plane in which an opening for unobstructed view can be made in the floor of the observer's cockpit. They can be quickly pointed in any desired direction, thus reducing to a minimum the necessary maneuvering of the plane, a real advantage when under attack by “Archies” or in working under adverse weather conditions.

For peace-time mapping work the hand-held camera, when equipped with spirit-levels on top, and when worked by a skilful operator, possesses some advantages over anything short of an automatically stabilized camera. For experimental testing of plates, filters and various accessories, the ready accessibility of all its parts makes the hand-held camera the easiest and most satisfactory of instruments.

The limitations of the hand-held camera lie in its necessary restriction to small plate sizes and short focal lengths, and in the fact that it must occupy the entire attention of the observer while pictures are being taken—the latter a serious objection only in war-time.

Essential Characteristics.—In addition to the general requirements as to lens, shutter and magazine, common to all aerial cameras, the hand camera must meet the special problems introduced by holding in the hands, especially over the top of the plane's cockpit. An exceptionally good system of handles or grips must be provided whereby the camera can be pointed when pictures are taken, and held while plates are being changed and the shutter set. The weight and balance of the camera must be correct within narrow limits; the wind resistance must be as small as possible; the shutter release must be arranged so as to give no jerk or tilt to the camera in exposing.

As to the method of holding the camera, a favorite at first among military men was the pistol grip, with a trigger shutter release (Fig. [37]). Because of the size and weight of the camera the pistol grip alone was an inadequate means of support and additional handles on the side or bottom had to be provided for the left hand. Small (8 × 12 cm.) pistol grip cameras were used to some extent by the Germans (Fig. [42]), and a number of 4 × 5 inch experimental cameras of this type were built for the American Air Service (Fig. [37]). But the grasp obtained with such a design is not so good as is obtained with handles on each side or with flat straps to go over the hands. The camera balances best with the handles in the plane of the center of gravity. As to weight, no set rules are laid down, but experience has shown that a fairly heavy camera—as heavy as is convenient to handle—will hold steadier than a light one. The American 4 × 5 inch cameras described below weigh with their magazines in the neighborhood of twelve pounds.

Fig. 37.—Pistol-grip aerial hand camera.

Representative Types of Hand-held Cameras.—French and German hand-held cameras are essentially smaller editions of their standard long-focus cameras, and a description of them will apply to a considerable extent to the large cameras to be discussed in a later chapter. The English and American hand-held cameras are generally quite different in type from the large ones, which are used attached to the plane.

Fig. 38.—Diagram of French (deMaria) 26 cm. focus hand camera, using 13 × 18 cm. plates.

The French hand-held camera uses 13 × 18 centimeter plates, carried in a deMaria magazine, and has a lens of 26 centimeters focus. The shutter is the Klopcic self-capping type already described, and is removable. The camera body, built of sheet aluminum, takes a pyramidal shape. In Fig. [38], A is the shutter release and B the rectangular sight, of which C is the rear or eye sight. The complete sight may be placed either on the top or on the bottom of the camera. At D are the handles, sloping forward from top to bottom; E is a catch for holding the magazine; F is a door for reaching the back of the lens and the lens flap; G is a snap clasp for holding the front door of the camera closed; H is a ring for attaching a strap to go around the observer's neck; I is the lever which opens the flap behind the lens and releases the focal-plane shutter; J is a snap catch for holding the front door of the camera open.

The operations with this camera are three in number. Starting immediately after the exposure, the camera is pointed lens upward and the plate changed by pulling the inner body of the magazine out and then in; next the shutter is set; then the camera is pointed, and finally exposed by a gentle pull on the exposing lever.

The English hand-held camera (Fig. [186]). This differs from the French in the size of plate (4 × 5 inch), in the shape of the camera body, which is circular, and in the type of shutter, which is fixed-tension variable-opening. In the longer focus camera (10 to 12 inch) the shutter is self-capping, and the aperture is controlled by a thumb-screw at the side. In the smaller (6 inch) a lens flap is provided in front of the lens and the shutter aperture is varied by a sliding saddle and cord. The handles of the camera are placed vertical, instead of sloping as in the French. The shutter is released by a thumb-actuated lever. Double dark slides are used, as the multiple plate magazine has not found favor in the English service.

The German hand-held camera (Fig. [42]). The German hand-held camera is, like their whole series, built of canvas-covered wood, the body having an octagonal cross-section. It is equipped with the Ica shutter and uses the Ernemann six plate (13 × 18 cm.) magazine. The excellent system of grips by which the camera is held and pointed is an especially commendable feature. On the right-hand side is a handle similar to the French type, but carefully shaped to fit the hand. The left-hand grip consists of a long, rounded block of wood running diagonally from top to bottom of the side, with a deep groove on the forward side for the finger tips, while over the hand is stretched a leather strap, the whole aim being to provide an absolutely sure and comfortable hold on the camera during the plate changing and shutter setting operations.

Fig. 39.—Front view of U. S. aerial hand camera, showing lens flaps partly open, and details of tube sight.

United States Air Service hand cameras. The hand camera developed for the United States Air Service and manufactured by the Eastman Kodak Co. is made in three models, using the bag magazine, a two-compartment magazine, and roll film, respectively. The shutter is of the fixed (one or two) aperture variable tension type, built into the camera. A distinctive feature is the double lens flap, in front of the lens actuated by the thumb pressure shutter release (Fig. [39]). In the bag magazine camera the shutter is set, as a separate operation, by a wing handle, and a similar handle controls the tension adjustment. In the two-compartment type (Fig. [33]) the shutter wind-up is geared to the plate changing lever, so that but one operation is necessary to prepare the camera for exposure. In the film type (Fig. [34]) a single lever motion sets the shutter and winds up the film ready for the next exposure. After the last exposure of all the film is wound backward on its own (feeding) roller before removing from the camera. The film is held flat by a closely fitting metal plate behind, and by guides at the edges in front, an arrangement which with small sizes works fairly well although the exquisite sharpness of focus attainable with plates is not to be expected. The saving in weight made possible by the use of film in place of plates in metal sheaths is about three pounds per dozen exposures.

In all these cameras the sight—a tube with front and back cross wires—is placed at the bottom. This position has been found the most convenient for airplane work, as it necessitates the observer raising himself but little above the cockpit, a matter of prime importance when the tremendous drive of the wind is taken into account.

CHAPTER VIII
NON-AUTOMATIC AERIAL PLATE CAMERAS

The ideal of every military photographic service has been an automatic or at least a semi-automatic camera, in order to reduce the observer's work to a minimum. Yet as a matter of fact almost all the aerial photography of the Great War was done with entirely hand-operated cameras. The primary reason for this was that no entirely satisfactory automatic cameras were developed, cameras at once simple to install and reliable when operated. Even the propeller-drive semi-automatic L type of the British Air Service was very commonly operated by hand, for many of the pilots and observers regarded the propeller merely as another part to go wrong.

Any automatic mechanism in the airplane must work well in spite of vibration, three dimensional movements, and great range of temperature. The requirements were well recognized when the war closed, but had not yet been met. Careful study of the conditions and needs by competent designers of automatic machinery may be expected to result at an early date in reliable cameras of the automatic type, but the description below of hand-operated cameras really covers practically all the cameras found satisfactory in actual warfare.

General Characteristics of Hand-operated Cameras.—As distinguished from the hand-held cameras the larger hand-operated cameras are characterized by the greater focal length of their lenses, the size of plate employed, and the manner of holding—by some form of anti-vibration mounting attached directly to the fuselage.

Except for the early English C and E type cameras which called for 10 inch lenses and 4 × 5 inch plates, the general practice at the close of the war by agreement between the French, English and American Air Services, was for the use of 18 × 24 centimeter plates and for lenses with focal lengths of approximately 25, 50 and 120 centimeters. The English also made use of a 14 inch (35 centimeter) lens, and never made a regular practice of anything larger than 50 centimeters. The Germans and Italians restricted themselves to the 13 × 18 centimeter size of plate, while a lens of 70 centimeters focal length was standardized with the Germans, in addition to the 25, 50, and 120 centimeter.

The particular focal length was determined by the nature of the photographic mission. Where large areas were to be covered at low altitudes or without the demand for exquisite detail, the shorter focus lenses suffice. The most commonly used lens in the French Service was the 50 centimeter, while the 120 was employed when high flying was necessary or when minute detail was required. As already mentioned, the common practice was to keep cameras of all focal lengths available, but the ideal at the close of the war was to have the camera nose and lens a detachable unit, so that any focal length desired could be secured with the same camera body.

The standard French camera. The hand-held form of French camera has already been described. The cameras for larger plate sizes and longer focus lenses differ only in the addition of a Bowden-wire distance release for the shutter and in the use of the Gaumont magazine which operates without the necessity of pointing the exposed side of the magazine upward. Fig. [40] illustrates the 50 centimeter camera, and Fig. [41] the 120.

Fig. 40.—50 centimeter deMaria hand operated camera on tennis ball mounting.

Fig. 41.—120 centimeter deMaria camera.

The German Ica cameras. These are larger editions of the light wood hand camera already described, but with the addition of a Bowden-wire shutter release. The body of the larger cameras carries a distinctive feature in the distance control of the lens diafram, worked by means of a lever which actuates racks, pinions and connecting rods leading to the lens. On the side of the camera body a shallow box is provided for carrying the color filter in its bayonet joint mount to fit on the lens (Figs. [42] and [43]).

Fig. 42.—German aerial cameras.

Fig. 43.—Diagram of German 50 centimeter camera.

Fig. 44.—U. S. hand-operated aerial camera (type M) with 10 and 20 inch cones.

The hand-operated bag-magazine camera of the United States Air Service (Type M) is similar to the small hand-held camera, but differs in three respects: a removable shutter (of the variable-tension fixed-aperture type) embodying an auxiliary curtain for capping during the setting operation; a Bowden-wire shutter release; and the employment of a set of standard interchangeable cones to hold lenses of several focal lengths. The 20 inch and 10 inch cones are shown in Fig. [44]. The operation of this camera is similar to the French standard cameras, but not so simple because of the number of motions required in manipulating the bag. Its chief objection for war work lies in fact in the magazine, which should be superseded by a two-compartment or other satisfactory type of plate changing chamber. The camera alone, with 20 inch cone, weighs approximately 40 pounds; the loaded magazine, with its plates in metal sheaths, 15 pounds.

Fig. 45.—English C type aerial camera.

The English C and E type cameras. The C and E type cameras have now chiefly an historic interest. They were the first used in the English service, fixed to the fuselage, and were later used in training work in England and in the United States. They were never built for plates larger than 4 × 5 inch nor for lenses of more than 12 inch focus, a limitation set by the lenses available at the time of their design.