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A METHOD FOR TESTIXCx PHOTOGRAPHIC LENSES 81
ment the photographic lens functions like an ordinary projection lens with the single but important difference that a plate of diffusing material is used between light source and object in order to avoid the danger of drawing false conclusions due to an incompletely filled lens aperture. This necessary condition has often been neglected in similar tests in the past and it cannot be too strongly emphasized here.
With this arrangement completed the test is conducted by taking separate and consecutive photographs of the hole in the test plate, setting it first on the axis of the lens and subsequently moving it at right angles to the axis in steps of any desired amount until a distance has been reached equal to half the length of the diagonal of the picture the lens is required to cover. After the focusing has been perfected with the hole in the center of the field the focus adjustment is not touched during the complete test. In the case of motion pictures the length of the diagonal is 28.65 mm. In carrying out the test the time of exposure will have to be increased as the hole is moved farther from tht central position for reasons connected with the construction of thf lens to some extent, with character of the performance of the lens and. more than anything else, with the character of the illumination. The difference in time of exposure thus observed is of practically no significance in reaching conclusions as to the performance of the lens. It should be self-evident that the image of the hole in the test plate will travel transversly as the test plate is moved going in the opposite direction and by an amount equal to the shift of the test plate multiplied by the magnification of the lens. This requires, therefore, that means be provided for moving the plate or film holder in a plane exactly perpendicular to the lens axis in order to make the various exposures.
The advantage of this test lies in the fact that all the aberrations of the lens manifest themselves on such a magnified scale that their effects are very large in comparison with the size of the grain of any photographic emulsion. If we choose a projection distance equal to a hundred and one times the focal length of the lens measuring from the front of the lens the magnification will be one hundred times. Interchanging object and image, that is reverting to the usual order, the reduction would be one hundred times or the magnification would be 1/1 OOx. The ratio between the two magnifications will be 10,000. The effect of residual aberration will therefore be many times as great (but not 10,000 times) in the proposed procedure as in the ordinary use of the lens.
Any kind of photographic material desired may be used and the color of the light may be made anything from blue to red by means of suitable filters placed between the light source and the test plate.
The chart reproduced herewith, Fig. 2, shows the results of a test carried out in this manner with a 50 mm. Raytar lens at full aperture (f : 2.3) together with tests of four other well known types of lenses of the same focal length and approximately the same relative aperture. Differences in lens performance are detectable in the original prints with an ease and certainty unapproachable by the ordinary