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A METHOD FOR TESTING A PHOTOGRAPHIC LENS
Dr. \V. £. Ray ton, A. S. C*
IT IS possibly no more difficult to design a photographic objective than it is to determine completely and accurately the value of the result. It seems obvious that since photographic lenses are designed as instruments for photography it should be a simple matter to take a picture with a lens and thus learn completely and almost without effort all about the ability of that lens to make a picture. This seems reasonable until the process is analyzed whereupon the photograph will be discovered to be not only a test of the lens but also something of a test of the camera, of the photographic emulsion, and of the technique of the photographer and the laboratory man. Since most people fail to derive much mental satisfaction from the contemplation of a negative, the process usually involves making a print also and in motion picture practice this positive print is projected on a screen entailing another series of variable factors all of which affect the degree of excellence of the final result.
There is another way in which lenses may be tested. By means of equipment known generally as a lens bench the magnitude of the residual aberrations in a lens can be measured. The information given by the lens bench is useful to the lens designer and it permits some attempt at quantitative specification of performance but it is practically impossible for the imagination to transform lens bench measurements into a mental picture of the quality of the image the lens can form. Lens bench tests can prove whether two lenses are alike or not. If they appear alike in every aspect they will undoubtedly perform alike in practice under the same conditions but if they are recognizably unlike under the lens bench tests it is generally impossible to determine what effect the recognized differences will have on the image quality of the lens without resorting to photographic tests.
The situation, therefore, is not all that could be desired even by one who has occasion to make comparative tests of lenses for his own information only and it is much more difficult for any agency such as a testing laboratory, for example, to find means for accurately and completely determining and expressing lens performance.
In the testing of any commodity there is apt to be a slight degree of suspicion on the part of the practical man concerning laboratory methods, a suspicion which like all others is founded on ignorance. It is manifested by the farmer in connection with the Department of Agriculture's tests of agricultural products and by the manufacturer
Director of Scientific Bureau, Bausch and Lomb Optical Company
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