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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.