In the tests devised by the Film Unit and in most of the research on identification of aircraft, an underlying problem was repeatedly encountered, the problem of the appropriate use of pictures. The majority of these pictures were photographic—a category which includes both pictures and motion pictures—but some were artificial in the sense that they consisted of nonsense shapes or highly schematic objects. The rule states that the observer of a picture must duplicate the visual angle of the camera that took the picture. In all probability, too great a strain can be put upon the compensating ability of visual perception at extreme departures from the “natural” visual angle subtended by a picture. Motion pictures added enormously to the possibilities but certain basic limitations were.