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Projection Lenses — Rayton 107
apertures which are less than those required in projection objectives. If an anastigmat be increased in diameter to give the relative apertures required of motion picture objectives the increase in spherical zones becomes so great that there is very perceptible deterioration of definition in the very center of the picture. The Petzval construction is characterized by small spherical zones and for this reason has been the favorite lens for the projection of motion pictures since the beginning of the art.
There has developed recently, however, a desire to reduce the projection distance below the previous average value without correspondingly decreasing the size of the projected picture. This will lead either to pictures whose margins are very poorly defined or else to the adoption of an anastigmat lens. If the latter expedient be resorted to, either the relative aperture of the lens must be decreased or less sharp definition be accepted over the whole area of the picture than is now expected. If the relative aperture be reduced in the interest of definition then illumination must suffer unless light sources of very high intensity become available. Another difficulty may be presented in the increased difficulty of getting an appearance of even illumination because of the greater angle of incidence on the screen of the pencils of light forming the marginal image. It is very doubtful whether the advantages which follow from the use of the very short projection distance are not overbalanced by the disadvantages in the quality of the projected picture.
In the early part of the paper it was promised that reference would be made later to some recent lenses which differ from the Petzval type of lens. Two patents have been issued which describe lenses very similar to each other, one of which claims decreased cost of production and the other increased illumination. These lenses, both of which are on the market, differ from the Petzval construction in that the back component is relatively close to the film gate. It may be argued, however, that the difference in construction between these lenses and the Petzval is more apparent than real. Referring again to Fig. 1, the front component is seen to resemble very much a telescope objective. As a photographic or projection objective, a telescope objective would be unsatisfactory because of its very Hmited field of sharp definition and because of its insufficient relative aperture. By adding to the telescope objective a second component, however, the combined focal length can be very much decreased thereby gaining the necessary speed and, further, by a suitable choice