_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.