It will be recognized that these plate sizes are chosen from those in common use before the war. A similar observation holds with even greater force on the question of plate _shape_. Current plate shapes have been chosen chiefly with reference to securing pleasing or artistic effects with the common types of pictures taken on the ground. These shapes are not necessarily the best for aerial photography. Indeed the whole question of plate shape should be taken up from the beginning, with direct reference to the problems of aerial photography and photographic apparatus. A few illustrations will make this clear, taking Fig. 17 as a basis. If it is desired to do spotting (the photography of single objectives), the best plate shape would be circular, for that shape utilizes the entire covering area of the lens. If it is desired to make successive overlapping pictures, either for mapping, or for the production of stereoscopic pairs, a rectangular shape is indicated. If the process of plate changing is difficult or slow, it is advisable, in order to give maximum time for this operation, to have the long side of the rectangle parallel to the line of flight (indicated by the arrow). If economy of flights is a consideration, as in making a mosaic map of a large area, it is advantageous to have as wide a plate as the covering power of the lens will permit. Reference to Fig. 17 shows that this means a plate of small dimensions in the direction of flight. If the changing of plates or film is quick and easy, the maximum use of the lens's covering power is made by such a rectangle whose long side approximates the dimensions of the lens field diameter. This is in fact the choice made in the German film mapping camera (Figs. 61 and 63), whose picture is 6 × 24 centimeters. An objection to this from the pictorial side, lies in the many junction lines cutting up the mosaic. Another objection, if the plane does not hold a steady course, is the failure to make overlaps on a turn. (Fig. 62.) Here as everywhere the problem is to decide on the most practical compromise between all requirements. [Illustration: FIG. 17.—Possible choices of plate shape.] =Focussing.=—The process of focussing aerial cameras was at first deemed a mystery, though undeservedly so. A belief was long current that “ground” focus and “air” focus differ. In other words, that a camera focussed upon a distant object on the ground would not be in focus for an object the same distance below the camera when in the plane. Belief in this mysterious difference went so far that certain instruction books describe in detail the process of focussing a camera by trial exposures from the air.