In connection with an examination of lenses to be used in airplane topographical mapping, a careful study has been made of the distortion of a large number of high-grade commercial photographic objectives. The apparatus designed for the measurement of distortion for infinite or finite object distance is described. The data show the distortion of the symmetrical anastigmat when the object is at an infinite distance, and also at unit magnification. At unit magnification, with a symmetrical lens, there should be no distortion, but tests of nominally symmetrical anastigmats show sufficient distortion to affect the performance of the lens. This distortion arises from lack of complete symmetry in the lens construction. Data for the distortion of the unsymmetrical anastigmat (Tessar type) are presented. The results show that rather large variations exist among individual lenses of nominally the same construction. The case in which the axis of the lens is bent, a result of slight prismatic action of the system, is discussed, and a method for determining the magnitude of tilt is described.
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