Motion picture projection; an elementary text-book (1921)

MOTION PICTURE PROJECTION 131 a very short focus lens or a comparatively small picture with a relatively long focus lens. The thing to avoid is extreme or abnormal conditions because the best result can be obtained only by being careful that each factor having an influence upon the quality of the picture is normal and efficient. Most important of these is Gundlach Projection Lenses which insure uniform definition with a brilliant image and the utmost luminosity. We differentiate between brilliancy of the image and the working aperture of the lens or the amount of light it collects and transmits because the former is determined by its color correction which if good, will preserve the contrast of the film and if poor, will flatten the image while luminosity is merely the inevitable result of making the diameter large in proportion to the focal length. Three principal factors govern the illumination of the picture, first the light source including its adjustment, current consumption and condenser system by which the film is illuminated. Next is the working aperture of the projection lens or the ratio between its diameter and focal length. The third is the size of the picture or its surface area. The working aperture of the lens is the only one in which we are directly interested. This ratio in Gundlach Projection Lenses is carried out to the highest degree with resulting apertures of F.2. to F.3.5. according to the focal length is not being practical for many reasons to maintain a uniform aperture of F.2. That the size of the picture is an important consideration is evident as it must be clear that the