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108 Transactions of S.M.P.E., July 1927
of shapes and glasses of the lenses of the rear component and its distance from the front lens, the useful field of view can be extended. Such, in fact, may be said to be the principle of construction of the Petzval lens. The front component differs somewhat but not greatly from a telescope objective; the back component adds to the light gathering power and to the field of view. Viewed in this light, the new lenses the type of which is represented in Fig. 4 do not differ greatly from the Petzval. From the standpoint of performance, nothing is claimed for the one while for the other it is claimed that it leads to a brighter image. This can be true, however, only if it can be shown that the construction makes it possible to produce lenses of higher relative aperture than other types and that the angular aperture of the illuminating sj^stem is large enough to make use of the enlarged aperture of the projection lens. Otherwise the mere fact of making the back focus short or, in other words, of bringing one of the components close to the film gate cannot have any effect on brightness of image.
DISCUSSION
Mr. Townsend: As I understand Mr. Raj^on, this latter type of lens theoretically does not give more light, but most projectionists will say that it does give considerably more so as to be perceptible to the eye. I wonder how that could be explained.
Mr. Rayton: In photometric comparisons of these lenses, all of which were reduced to the same relative aperture, we found no difference. A slight difference will exist in the case of a lens whose back component is cemented as compared with one whose back component is not cemented because of loss of light at the uncemented surfaces. When the lenses are diaphragmed to the same relative aperture, there is no other difference.
Mr. Egeler: With the objective and 16 mm. film the projection distance is often a quarter or a fifth that of the 35 mm. film. Should it not be possible to use certain types of construction for the smaller film which would allow the large aperture because the ratio in film width is 2: 1 whereas projection distance would of the ratio of 4: 1 or figures of that order?
Mr. Rayton: As a rule, the angular field in projecting the half size film is at least as great as in ordinary theater projection of full size film. While special lens constructions may be in use in amateur work, I was not thinking of that type of projection.