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 elminate 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.