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. CHAPTER IV LENSES FOR AERIAL PHOTOGRAPHY