Journal of the Society of Motion Picture Engineers (1930-1949)

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500 WADDELL November used in preference to an //2.0 lens stopped down to//5.6. Of course, increasing the focal length of the lens accomplishes the same effect as reducing the aperture. The final objective in this direction is obtained when using a nearly parallel beam of light through the prism. This can be accomplished by a double-lens system where the objective lens forms an aerial image ahead of a second lens; the second lens focusing this image at the film plane. The second lens should be 1.4, o I .45 2£ 5' 7i 10' I2£ 15' MAX. ANGLE Or INCIDENCE (I) Fig. 3 focused so as to project the aerial image at a 1: 1 magnification. In such a system it is comparatively easy to introduce a reticle at the plane of the aerial image if such is desired. The rotating prism acts as a shutter as well as an image-compensating device. In acting as a shutter, as was explained earlier, the time of exposure is dependent on the maximum angle of incidence. If the prism does not run true, a jumpy picture would be obtained and therefore, the housing of the prism shaft must be of sufficient strength j