=The Question of Plate Size and Shape.=—Plate size is determined by a number of considerations, scientific and practical. If the type of lens is fixed by requirements as to definition, then the dimensions of the plate are limited by the covering power. From the standpoint of economy of flights and of ease of recognizing the locality represented in a negative, by its inclusion of known points, lenses of as wide angle as possible should be used. If the focus is long, this means large plates, which are bulky and heavy. If the finest rendering of detail is not required a smaller scale may be employed, utilizing short focus lenses and correspondingly smaller plates. Thus a six inch focus lens on a 4 × 5 inch plate would be as good from the standpoint of angular field as a 12 inch on an 8 × 10 inch plate. This is apt to be the condition with respect to most peace-time aerial photography, which may be expected to free itself quickly from the huge plates and cameras of war origin. For work in which great freedom from distortion of any sort is imperative, small plates will be necessary, for two reasons. One is that the characteristic lens distortions are largely confined to the outlying portions of the field. The other is that a wide angle of view inevitably means that all objects of any elevation at the edge of the picture are shown partly in face as well as in plan, which prevents satisfactory joining of successive views (Fig. 128). In making a mosaic map of a city, if a wide angle lens is employed with large plates, the buildings lying along the junctions of the prints can be matched up only for one level. If this is the ground level, as it would be to keep the scale of the map correct, the roofs will have to be sacrificed. In extreme cases a house at the edge of a junction may even show merely as a front and rear, with no roof, while in any case the abrupt change at these edges from seeing one side of all objects to seeing the opposite side is not pleasing. The table in a preceding section gives the relation of plate size to focal length found best on the whole for military needs. Deviations from these proportions in both directions are met with. In the English service the LB camera, which uses 4 × 5 inch plates, is equipped with lenses of various focal lengths, up to 20 inches. The German practice, as well as the Italian, was almost uniform use of 13 × 18 centimeter plates for all focal lengths. Toward the end of the war, however, some German cameras of 50 centimeter focal length were in use employing plates 24 × 30 centimeters.