USING A HAND CAMERA FOR OBLIQUE SHOTS. The observer-gunner of an army aircraft on reconnaissance is seen above taking oblique photographs with a hand camera. For such shots the camera lens is pointed between fifteen and thirty degrees below the horizontal. A lens of medium focal length is generally used.
THE limitations of visual reconnaissance, due to speeds and heights at which aircraft travel and to the lengthy training necessary before observers can be relied upon to discern and identify the important and to disregard the unimportant, make it expedient that trained R.A.F. staff officers, able to study air photographs methodically, and at comparative leisure, and in conjunction with all the other information available, should be the men to draw the vital conclusions as to enemy’s dispositions and intentions.
Air photography for military use was already of great assistance in the war of 1914-1918, and in the intervening years calls have been constantly made on the inventive genius of the scientific world, on the camera designer, the optician, the chemist, the electrician, and such practical technicians as the fast-working press-photographer — each to play his part in the perfection of apparatus and processes, and to promote speed, accuracy and economy to the greatest degree.
From clumsy plate cameras and slow, improvised dark-room practices have been evolved highly efficient electrically operated film cameras, with special lenses, light filters, shutters, and so on, modern mobile dark-rooms fitted with the latest automatic devices to ensure perfect and speedy production of both negative and prints, and self-contained lighting, heating and refrigerating units ensuring efficiency in any climate.
The pursuit of detailed and extensive knowledge of enemy resources and dispositions must inspire all the important activities of the air photographer.
INSIDE A MOBILE DARK ROOM. The R.A.F. employs large numbers of mobile dark rooms for the rapid processing of films in the field. These are fitted with self-contained lighting, heating and refrigerating units ensuring efficiency in any climate. The compact layout of one of these vans is shown here.
Personnel of the R.A.F. who are intimately connected with air photography can be classified under three headings. First there are those whose duty it is to navigate the aircraft over the area that is indicated, and there to take the photographs. Second comes the technical personnel on the ground, performing subsequent dark-room processes and producing the finished prints, being also responsible for the maintenance of all apparatus. Finally there is the interpretation personnel, whose work consists of examining each print, extracting from it the fullest relevant information, and passing this information to the staffs and departments concerned in an appropriate and intelligible form.
Before going into further detail let us consider the main purposes fulfilled by air photography, since many different demands on procedure are made. Naturally to the three fighting Services speedy and accurate information of enemy activities is vital. Again, the Ministries of Economic Warfare, of Shipping, of Home Security, and so on, can gain valuable information from air photographs, their demands in this respect being far from infrequent.
The main purpose of air photography is for reconnaissance work. An army commander today is reluctant to contemplate any operation unless he has the most accurate and recent information possible of enemy terrain and enemy disposition, actual and potential, relevant to the occasion. Again, the information which is vital to naval operations regarding numbers and types of vessels in enemy harbours, concentrations at sea, the effective state of maintenance and repair facilities at port, and so on, can often be obtained with accuracy and speed by means of air photography.
To the Air Force itself early information as to the results of bombing raids, for example, obtainable mainly by means of air photography, is essential to the most efficient and economical concentration of further activities.
Mapping from air photographs is another important development, whether it be for the study of hitherto uncharted territory over which operations are contemplated, or for filling in additional detail on maps which already exist.
As soon as details of a required photographic mission are received, an aircraft crew is selected, decisions are made as to what type of camera with what focal length of lens and with what light filter, is to be carried, how many exposures may be required, and what procedure should be adopted. All available maps and other means of identifying the objective are examined, and a weather report is obtained. The intelligence officer is consulted as to the probable intensity of hostile interference and how it may best be circumvented. While final decisions are being made, the fitting in the aircraft of the camera and its releases and sights goes on. The testing of each piece of apparatus as it is installed is a necessary precaution.
The actual taking of the photographs is not less responsible a job than is their subsequent careful processing. It may be necessary to manoeuvre about while clouds clear, and to evade hostile aircraft, until, photograph by photograph, pilot and crew have achieved their mission.
As soon as the aircraft lands the exposed film is handed to members of the ground technical staff who hurry off to the dark rooms with the precious magazines. Here all has been prepared; developing and fixing solutions have been-made up, processing tanks filled, washing and drying appliances are ready to receive each film negative at the appropriate stages, and printing machines waiting to rush off, as may be required, either first proofs or large quantities of prints. Both speed and efficiency are essential, and members of the section work in teams so that no time is lost. In the case of reconnaissance photographs, a few prints from each negative are soon placed in the hands of members of the “interpretation” staff who, in their turn, are holding themselves in readiness to carry smoothly forward the sequence of their duties.
RUSHING OFF THE FIRST PRINTS. The aircraft has returned with its precious exposures. These are hurried off to the dark rooms where everything is ready. They are quickly developed and dried on large drums (shown left) …
… after which prints are made and sent to the interpretation staff for detailed examination.
At first sight the advantages of such specialized team work may not be apparent. It is, however, no longer drummed into us that the camera cannot lie, and even though many of the objects depicted — buildings, railways, rivers, roads and so on — are features familiar to all, it should be remembered that photographs suffer from the fundamental limitation that objects of three dimensions are being recorded on a two-dimensional surface. We know, on looking at a photograph taken on the ground in the plane normal to human vision, that the image of a man standing in front of a house, for example, is not just a flat object in the same plane as the wall of the house, nor is the house itself without the third dimension that nowhere actually exists on the surface of the print. Our innate knowledge and imagination, however, automatically rouses a sub-conscious appreciation of the third dimension, which in direct human vision is made evident by our two eyes each seeing actual objects from a slightly different view point. The effectiveness of this is most marked at close range. For obvious reasons the majority of air photographs have to be taken vertically from great heights, and objects are seen from a normally unfamiliar point of view. Thus only the roofs of buildings may be seen, trees appear as round objects with nothing beneath, and troughs of valleys appear to be no further distant than the crests of hills. These generalizations must be modified to some extent as regards sunny days with the sun fairly low in the sky, and long shadows accentuate the mind’s perceptions of the third dimension.
A correct or closely approximate appreciation of this third dimension is necessary for obtaining the fullest information about the details of an area under review, and all the specialist skill of the photographic “interpreter” is called into play, whether it be by means of pairing photographs taken from slightly different viewpoints for stereoscopic viewing, by examining shadows, tracks, and so on, or by other intuitive methods. This problem is dealt with later in this chapter.
There are a thousand and one details which might well escape the notice of the uninitiated, but which enable an interpreter to draw definite conclusions. Nor will he allow himself to be misled by any methods which the enemy may have adopted to deceive the ordinary “reader,” such as camouflage, substitution of dummy objects for real ones, and other similar devices.
In passing, reference may be made to the necessity for correct assessment of scale, and therefrom of distance between objects depicted on the photograph. Minute variations of the camera axis from the vertical, due to uncontrollable movement of the aircraft, can cause appreciable variations of scale in different parts of the same photograph. All these must be determined for highly accurate measurements to be made.
So, armed with the report furnished by the pilot who undertook the mission, with maps, instruments, previous records and other information the interpretation staff get down to work. As soon as they are to hand, all photographs are quickly identified from maps or previous knowledge, and a quick selection is made, discarding prints or negatives which may be superfluous. The total number of copies required from those selected is determined by reference to the original order and then, quoting the serial registration numbers which are automatically recorded on each, instructions are speedily sent to the ground technical staff to run off the required number of prints with the least possible delay.
INTERPRETATION STAFF AT WORK. This photograph shows the photographic branch at Bomber Command where experts are busy examining the latest batch of photographs received from an aircraft just back from an enemy area which is to be bombed.
Meanwhile, a more detailed examination of the first prints of the photographic mission is taking place. Compared as necessary with photographs of the same area previously taken, changes in detail, due, maybe, to raids, to movements of shipping at ports, to new military or industrial construction, and so on, are assessed.
As each piece of important information comes to light it is marked by conventional signs, and, after careful checking against other available data, noted on a memorandum. Although photographs are generally taken with a view to studying some clearly defined operation, the interpreters frequently light upon valuable indications of enemy activity in hitherto unsuspected directions.
When each photograph has been thoroughly combed for information, what has been gleaned is noted on duplicate copies as they stream in from the printing section, points of individual importance being emphasized to suit each recipient. Then by quickest route, the annotated prints, together with such explanatory notes as are necessary are distributed to the various headquarters and departments where all the information they reveal plays a very large part in the issue of operation orders and other action.
Height of Objects
It would be well to mention here how it is that the height of objects in vertical air photography can be appreciated. It is, of course, only in the centre of an air photograph that are recorded the absolutely vertical light rays reflected from objects on the ground. Three-dimensional objects which appear towards the edges of the photograph are recorded by rays which have a minute, though appreciable obliquity, and consequently there is recorded in the case, for instance of a building, a minute oblique portrayal of the wall or walls which appear as being nearest to the centre of the photograph.
STEREOSCOPIC VIEWING DEVICE. When stereoscopic pairs of photographs are viewed simultaneously through a simple optical device, like that shown here, by means of which an appreciation of the third dimension can be obtained, objects photographed appear to stand out in relief.
In the case of normal human vision of actual objects the separation of the eyes, each, therefore, seeing from a slightly different point of view, promotes an unconscious appreciation of the third, less obvious dimension. Thus it is found that if two consecutive air photographs are taken with perhaps a slightly exaggerated separation rather more than proportionate to the height of the aircraft, a similar appreciation is obtainable stereoscopically, a simple optical arrangement enabling the two photographs to be viewed simultaneously, one only by one eye and the other by the other. This method is equivalent to looking at the earth through the two eyes of a giant. The appropriate separation is roughly that needed to give a two-thirds overlap. Too great a separation gives too exaggerated an effect to heights, and too small a one a quite inadequate indication. Limitations imposed when applying this principle to oblique photographs are obviously so great that very little advantage has ever been taken of it.
By far the larger number of air photographs taken by the R.A.F. are vertical photographs, that is, taken with the lens of the camera pointed directly downwards and providing valuable and informative views of the ground in plan.
Nevertheless, there are occasions when photographs giving an immediate appreciation of the heights of ground features are of extreme importance. Such photographs, which in appearance are similar to those taken from the tops of high hills, can be obtained if the camera is mounted in a low flying aircraft so that its axis is slightly depressed from the horizontal. The photographs are “obliques”.
The comparative advantages and limitations of each type are borne in mind by the authorities when detailing photographic missions.
The vertical photograph has the advantage that it can be usefully taken from any height, however great, thus reducing the chances of any hostile interference.
By fitting lenses of longer or shorter “focal length” — the distance, that is, between the focal point of the lens near its centre, and the centre of the negative behind it — the “scale” of the photograph — expressed as a simplification of so many miles on the ground to so many inches on the photograph, or as a representative fraction — can easily be decreased or increased as may be desired for any given height, the area of ground covered on plates of exactly the same size being, of course, at the same time, proportionately decreased or increased.
As an example, if an aircraft flying at 20,000 feet uses a camera of standard film dimension 7 in. by 7 in. fitted with a lens of 14 in. focal length, the scale of the resultant photograph expressed as a representative fraction will be 1/17143. In other words 1 inch on the photograph represents 17,143 inches (or roughly 1,430 feet) on the ground, and the area of ground covered by the whole negative is seven times 1,430 feet square, or 10,000 feet by 10,000 feet. By substituting a lens of 20 inches focal length, the scale is increased to 1/12000 but the area covered reduced to 7,000 feet by 7,000 feet. If a lens of only 5 inches focal length is used, the scale is reduced to 1/48000 but the area covered increased to 28,000 feet square or just over twenty-eight square miles.
Obviously the smaller the scale of photographs taken from the air the more difficult it is to distinguish and identify small objects on the ground, and the solution is not to be found in enlargement from the negative because the grain in its emulsion, also proportionately enlarged, may itself be in the region of as much as 1/200th of an inch.
If, therefore, a wealth of accurate detail is needed, as for instance in assessing bombing results on specific objectives or in identifying small craft in harbours, it is necessary to use a lens of long focal length, and make a correspondingly greater number of successive exposures, to cover a given area of country. On the other hand, for such a purpose as the preliminary examination of large tracts of country in which the enemy is suspected of establishing new aerodromes, for example, it is easier and more economical to use a lens of short focal length. Oblique photographs by way of comparison, can normally only be taken from low altitudes — say, for instance, from 1,500 to 3,000 feet — with a lens of medium focal length, and in a camera which is aimed at an angle of fifteen to thirty degrees below the horizontal.
Difficulties such as those involved in efficient mounting preclude the adoption of lenses of long focal length and higher flying. Oblique photography, therefore, is usually confined to special objectives, or territory where the likelihood of serious hostile interference is remote. A typical instance of where obliques may well be of more use than verticals, suggests itself in fast moving desert warfare, where photographs from several angles of forts, and of strong points on escarpments or in defiles can be of immense value in determining what tactics should be adopted and even what special equipment is necessary for an attack.
Easily understandable, these photographs can be supplied without delay, even by message dropping to the army units engaged in the actual operations, the information given being immediately utilized to best and utmost advantage.
The measurement of distances between objects is far easier in a vertical than in an oblique photograph. In the case of the former unless there was any abnormal deviation of the camera axis from the vertical, the scale can usually be regarded as constant all over the photograph, whereas in the latter the scale progressively decreases from the foreground at the bottom of the print until the horizon near the top is reached, a fan shaped area of country being restricted to a rectangular photograph. If, with an oblique, the angle of depression of the camera axis at the moment of exposure is accurately known — though this is extremely difficult to determine — it is possible in conjunction with other data, to construct what is called a “graticule,” which will enable distances to be calculated within fair limits, but the process is a long one and. is therefore rarely adopted in normal service work. Consequently, an air photographer when carrying out oblique photography always endeavours to get his main important objectives in the centre foreground of each photograph so that a general estimate of heights and distances adjacent thereto may be fairly easily estimated within limits, by comparison of some of the features with known similar features.
It will readily be realized that if successive adjacent vertical photographs are accurately taken from a consistent height, it is possible to piece them together, a part only of each, in “mosaic” form, and so to produce a photographic map of wide stretches of country. In practice, however, minor distortions are almost certain to occur and therefore the accuracy of such mosaics can at best be only approximate.
The strategic and tactical requirements of the Royal Navy and the Army make heavy demands upon the air photographer. Aircraft are the eyes of all the Services and the camera is by no means the least important attribute of the aircraft at their disposal. It is easy to picture the many objectives, mobile and immobile, of which both the Royal Navy and the Army can by air photography obtain speedy, accurate and vital information.
Quite independently, and within the Air Force itself, air photography plays a most important part in obtaining information on which are based plans for the utilization of its striking power with the utmost efficiency and economy in effort. Happy indeed is the bomb aimer who, from previous study of air photographs of his targets, knows that from whatever direction he may approach them he will have no difficulty in identifying them, and so ensure that his efforts achieve maximum efficiency. Information gained from air photographs as to the locations, strengths and types of enemy air forces assists in the most efficient disposition and employment of our own air forces, both fighter and bomber, to forestall or counter the air effort of the enemy, and so attain essential air superiority.
All bomber aircraft carry a camera with the object of recording photographically the place where the bombs have fallen. It is quite simple to do this by day, and a high standard of air photography in daylight was reached even during the last war. Recently, however, the art of night photography has been developed so successfully that it can be regarded as one of the principal means of ascertaining the effectiveness of the bomber’s work. As soon as the bombs are released the camera shutter is opened. The ground is, however, dark and nothing is recorded on the film except possibly the flashes caused by anti-aircraft gunfire or the glow of fires already burning in the target area. The bomb aimer then releases a flash-bomb which has a time fuse allowing it to fall a certain distance before it explodes, giving a flash of tremendous brilliance. The release of this flash-bomb is timed so that it will give its flash a few seconds after the bombs have exploded, thus recording on the film a picture of the ground which will include the smoke of the bomb bursts. Immediately after the flash the shutter is closed. From this photograph the exact place at which the bombs have fallen can be accurately plotted. In the latest cameras carried by British aircraft the whole of this process is automatic and is operated by the bomb aimer pressing a button immediately after the bombs have been released.
Camera Guns
Nor would any reference to the value of photography to our Air Force be complete without a brief mention of the part it plays in the preliminary training of its personnel in other activities. In his early training the fighter pilot armed with a camera gun takes a photograph of his mock adversary instead of firing a bullet at him; the bomber pilot photographs his objective in early training instead of dropping a bomb on it. From calculations based on the position of the image of the opposing aircraft or the bombing objective on the negative, it can readily be ascertained whether the bullet or bomb would have reached its mark and the nature and extent of any error determined.
CAMERA GUN IN ACTION. During their early training pilots and air gunners are taught to photograph their mock adversary instead of firing at it. Here the camera gun is seen in use with (inset) a developed photography showing a “direct hit”. A similar device is used for practice bombing.
It is obvious also that air photographs taken at home give an indication of how much will be revealed to the enemy of potential objectives should he carry out a photographic reconnaissance of them.
It will therefore doubtless be agreed that by whatever Service it is employed and into whatever sphere the information so gained is directed, air photographic reconnaissance plays a most vital part in gaining that knowledge which must result in strengthening our whole war effort.
The training of the air photographer in whatever capacity he may be employed must of necessity be thorough. One oversight, one mistake, and a vital piece of knowledge which may have far-reaching consequences is lost and effort proportionately weakened. The speed and accuracy which are demanded in the execution of all operational duties are therefore the keynotes of the training of the air photographer of whatever the category to which he belongs.
The flying personnel who may be selected for photographic duties are those who, in their normal air training have shown special qualifications in navigation and airmanship and also possess a very keen sense of initiative which will ensure that, whatever obstacles be encountered, nothing will deter them from their primary task of securing and delivering the required photographs.
A thorough knowledge of the working and manipulation of the photographic apparatus carried in the aircraft, cameras, sights, mountings, etc., is gained either at an appropriate training establishment or from the photographic instructor at a squadron, according to circumstances. Practice flights of progressive difficulty for single photographs, line overlaps and “mosaics” are carried out, and faults and remedies pointed out. A sufficient appreciation of the work of the technical and interpretation personnel must be gained so that, as far as possible, the flying procedure adopted may facilitate their subsequent work.
Trainees for ground technical duties may be either boy entrants who undergo a long ab initio course both theoretical and practical, or specially enlisted aircraftmen who have had previous experience of commercial photography in civil life. Whatever the capabilities of the recruits there is much to learn and unlearn in acquiring the essential knowledge of the special apparatus and methods which are best adapted to the requirements of a modern air force.
The interpretation personnel are selected as far as possible from men who have had previous experience in air photography. Quick perception, an eye for detail, and a logical mind are basic attributes.
Though the work is hard, there is no lack of applicants for this interesting pursuit, the more so as most posts carry commissioned rank on account of their onerous responsibilities.
However proud we may be of the wonderful achievements of our air photographers, it would be idle to pretend that finality has yet been reached in the design of apparatus, mechanically or optically, in technical processes or in the ancillary instruments and materials, all being necessary for efficient work.
The research laboratories, service and civilian, national and allied, are continuously working on the problems, mechanical, optical and chemical, the solution of which will most certainly increase the usefulness of photography from the air.
