OBLIQUE AIR VIEW of the city of Providence, Rhode Island, U.S.A. The camera in the aircraft is inclined at an angle of about twenty degrees below the horizontal to obtain the best effect. It is noticeable how well buildings, trees, roads, rails and waterways stand out in such photographs.
EVERYBODY has now become familiar with the bird’s-eye views aptly applied as illustrations in industrial advertisements, and with the air photographs used in the Press to show the results of major disasters such as fires and floods. In commercial spheres air views are used to illustrate factories, hotels, building operations, road construction, estate development and numerous other activities. The archaeologist finds these views invaluable in his researches, and more than one poster artist has found inspiration in them.
Oblique air views are useful in teaching geography, for they show clearly the influence of streams, hills and the like in shaping the development of cities. It is not only in the instruction of the young that air photographs are valuable - Parliamentary Committees and similar bodies are frequently called upon to study them in connexion with proposals to amend boundaries and with other problems of administration. Long before the days of aircraft, artists exercised their powers of imagination to present what they called bird’s-eye views of scenes of outstanding interest. The illustrated magazines of the later years of the last century are full of these views. The bird’s-eye view is valuable because the elevated viewpoint permits the main centre of interest to be seen, if not in its entirety, at least more completely than from any viewpoint at ground level. Not being able to see the wood for the trees is an old expression that well illustrates the point.
To get a good view of anything one must get away from it; too near an approach is apt to spoil the picture.
In the early days of photography a surprising number of experiments were made to test the possibilities of taking photographs from the air. Even before the days of dry plates, enthusiastic photographers took their cameras, with all the paraphernalia of wet collodion and silver nitrate, up into the sky in captive balloons. As long ago as 1858, Nadar, in France, obtained successful pictures in this way. Free balloons, kite balloons, pigeons and other carriers have been used to support the camera in the air and thus take advantage of an elevated viewpoint.
Balloon photographs taken fifty years ago compare favourably with the productions of today.
The coming of the aeroplane enabled the air-view operator to choose his view and his viewpoint instead of being limited in his choice by the wind and other factors inseparable from work in balloons. But the aeroplane brought with it the necessity for an advance in photographic technique.
The greatly increased speed of travel meant that exposures must be correspondingly short. The elimination of the effects of vibration made even more rigorous demands, and the buffeting of the slipstream imposed a handicap that called for a high standard of physical fitness and skill on the part of the air-view cameraman. The old type of camera with leather bellows was liable to be blown inside out like an umbrella in a gale and had to give way to a rigid type of construction. Air-view cameras have developed from mahogany boxes into precision instruments of aluminium. With the need for shorter and shorter exposures came the necessity for a more and more exact choice of the moment for pressing the trigger.
The speed of aircraft has increased so much in the past few years that photographic technique has not found it easy to keep pace. When aeroplanes flew at sixty miles an hour, the task of the air-view cameraman was not so difficult that failures were common, but machines flying as slowly as 60 m.p.h. are no longer manufactured. The cameraman today must work at speeds of about 120 m.p.h., which is roughly equivalent to 180 feet per second. Hence air photography has developed into an art similar to that of revolver shooting.
If the cameraman, buffeted by the wind created by his progress, and hampered with heavy and ungainly flying kit, misses the exact instant for pressing the trigger his picture is likely to suffer and he must try again. His task is not easy.
He must choose a time of day for his work when the sun is shining in a direction which gives bright illumination on what his client thinks is the most important part of the view. He must choose the most satisfactory altitude to yield a pleasing perspective and a distance from his target which will enable him to include in his picture just as much as he wants to show, no more and no less.
Having decided all these things, with due regard to flying regulations which prohibit any low flying over towns, he then has the somewhat unenviable job of coaxing the pilot to fly the aeroplane just were it is wanted. Some pilots are inclined to be temperamental, and patience, skill and good humour are necessary before a harmonious team of pilot and photographer can be created. Once they understand each other’s problems they can concentrate wholeheartedly on the job.
Unlike air photography for mapping purposes, nearly all air-view work is done on glass plates. For some reason that the chemists have not yet discovered it is easier to put a fast photographic emulsion on a glass plate than on a celluloid film. Moreover, it is easier to develop a batch of glass negatives without accidental scratching than to handle a similar number on celluloid.
A considerable degree of enlargement is generally necessary in air-view work, and glass negatives for various technical reasons yield superior results in this respect. The celluloid film has the advantages of convenience and lightness, but when a maximum excellence of performance is demanded the glass plate is considered to have overwhelmingly good qualities and it is in almost universal use for air-view work.
Among the air-view operator’s equipment, a good sighting device ranks high in importance. Different operators have individual preferences, but most sights in modern use are of either tubular or open-frame type, rigidly fitted to the camera body. A bead and cross-wire arrangement serves to indicate the centre of the field of view, but even this simple device needs to be accurately fixed so that the axis of the sight and the axis of the camera are parallel.
DARKROOM WORK on negatives taken in air cameras. Negatives rarely conform exactly to scale, but prints can be taken in such a way that they agree to the scale required. Divergencies in a negative can be corrected by altering and adjusting the distances between negative, lens and sensitized paper
It is of considerable importance that the extent of the area included by the lens on the negative should be accurately shown by the framework of the sighting device. Some operators have a preference for an optical sight giving a brilliant picture by the aid of a negative lens, but a bead and an open frame are perhaps in most common use. Even so small an accessory as the sight needs to be of robust construction to resist the many jars and jolts that are apt to occur.
Working in an open cockpit, the operator gets cold and his sense of touch becomes blunted; the confined space is awkward and the apparatus needs to be designed to stand up to rough handling.
In the choice of aircraft suitable for his work, the air-view photographer has many factors to consider. Not the least important is the question of expense. A small aircraft of the Moth or similar type is relatively cheap to operate, and as the cost of the flying plays a major part in the total expenditure, there is justification for seeking a machine of this type. Although it is pleasant to fly in a multi-engined enclosed cabin machine, such luxury costs ten pounds or more an hour, and it is only natural that small single-engined open-cockpit machines costing only half as much to run should be used when possible. The open cockpit is not altogether a disadvantage. It makes work a little more difficult because of the cold and the draught, but the window arrangements in cabin aircraft are seldom ideal for air-view work.
Sometimes the windows do not open conveniently, are not in the right place, or the framework restricts the view. In any event, they provide a complication. The essential characteristic of an aeroplane for air-view work is that the wings should be out of the way of the camera’s field of view. The amount of view obscured by the wings of some machines is unnecessarily large, and why such a bad measure of visibility is still tolerated in modern aircraft design is something of a puzzle.
The air-view operator talks of his photographs as “obliques”. The term is used to distinguish them from the “verticals” used for air survey photography. In air survey photography the axis of the lens points vertically to t he ground beneath the aeroplane, but in oblique work the axis of the lens is generally depressed twenty degrees or so below the horizontal. This characteristic makes the high-wing monoplane particularly suitable for oblique work, and in it the photographer is able to command two fairly wide arcs of unimpaired visibility - one to port and the other to starboard.
High Shutter Speeds
The “oblique” air camera is generally a much simpler affair than the air survey camera. All that is necessary for oblique work are a good lens, good bodywork, a reliable shutter and a plate holder. The lenses in common use are of focal length between six inches and ten inches and must be capable of giving perfect definition at large working apertures, f/3·5 or so.
The bodywork is nowadays of aluminium or aluminium alloy. It must be well balanced in weight and furnished with well-placed handles. The shutter is generally of the focal-plane type consisting of a fabric blind with a slit in it moving across the surface of the sensitive plate and in close proximity to it. Shutter speeds must be high enough to discount the speed of the aeroplane and the effects of vibration. A shutter speed of 1/250th part of a second is fairly normal. Some form of quick winding arrangement is provided for setting the shutter, and a trigger is provided for releasing it.
The camera is nearly always held in the hand at eye level, and its weight needs to be restricted as much, as possible. For this reason plate magazines and changing boxes are unsuitable. The size of plate in common use in Great Britain is 5-in by 4-in. Each unexposed plate is carried in a separate sheath, which can be dropped quickly into position at the back of the camera, exposed and then quickly exchanged for another. The type of sheath used most for this kind of work in Great Britain is the Mackenzie-Wishart envelope, known to every Press photographer for its convenience of handling.
AUTOMATIC AIR CAMERA used for survey work. The magazine at the top contains 200 exposures, size 8¾-in by 6½-in. The box at the right with the hinged lid houses the instruments, which are photographed with each exposure and show the aeroplane’s altitude and so forth. Electric leads transmit the required power from a battery or power unit in the aircraft. Exposures can be made automatically at any desired intervals of time.
Although the camera is most commonly held in the hand, there are circumstances where it is convenient to have it mounted on some kind of fitting rigidly attached to the aircraft. This fitting generally consists of a fork which is capable of rotation about the vertical axis.
The camera is mounted in bearings in the prongs of the fork so that the angle of depression of the axis can be varied. The camera can thus be trained oil the target in the same way as a machine-gun.
The hand-held method of securing air views persists largely because of its simplicity, cheapness and freedom from auxiliary equipment and unnecessary weight. These considerations, however, limit the scale and size of the picture. Lenses of longer focus working on larger plates give a larger scale or, alternatively, permit the camera to be operated farther from the target and still provide a picture of sufficiently large scale.
In addition, for some types of work it is desirable that a series of photographs should all be taken with the lens axis at the same angle of depression. With a hand-held camera variations are bound to occur.
In aiming the camera there is an advantage in exposing either before the target has been reached or just, after it has been passed, instead of when it is exactly opposite the aeroplane. The apparent speed of the target is then less and longer exposures can be made without spoiling the definition.
Range of 600 Miles
Although these “obliques” are used primarily as pictorial records they can be profitably used for producing small-scale reconnaissance maps of flat country.
In parts of Canada the system has been extensively used in the exploration of many thousands of square miles of lonely forest and lake country north of the prairies. When the flying height of the aeroplane and the focal length of the lens of the air camera are known it is possible - so long as the angle at which the axis of the camera is depressed below the horizontal is known also - to rule a network of lines on the photograph dividing it into areas, each representing (say) a quarter of a mile square on the ground. The transfer of the topographical detail from the photograph to the map grid is then made square by square by eye, and large areas can be completed expeditiously.
The progress of science has made it possible to obtain photographic records over immense distances, and the camera now has a greater range than the human eye.
The production of photographic emulsions sensitive to infra-red rays has resulted in the camera being able to pierce the haze and see distant landscapes that would be hidden from an ordinary camera or the naked eye.
The potentialities of this new technique in the air are remarkable, and excellent photographs at immense ranges have already been secured. The whole of the eastern counties of England from London to the Wash has been photographed on one negative, and in America mountains have been photographed at a distance of something like 600 miles.
The particular charm of the oblique air view is not merely the novelty of the viewpoint. The vertical survey photograph gives only a plan view which, of course, is its intention. For any artistic purpose, however, the photographing of one side or face of an object does not exhibit the full characteristics. The air survey photograph gives the top view, and photographs taken on the ground can provide a view of the front and the side of the object; but the oblique air view is unique because it is able to show the front, the sides, and the top of the object simultaneously. That is why oblique air views are so satisfying. The illustration at the top of this page is an excellent example of the qualities of oblique air views.
PRINT OF AN AIR VIEW of Durban, South Africa, showing the residential area, the harbour and the promontory in the distance known as the Bluff. At the same exposure the camera photographs the instruments contained in it, so that a record of the altitude, time, subject and number is incorporated on every negative.