Richardson's handbook of projection (1930)

1266 HANDBOOK OF PROJECTION FOR quickly changing from one projection lens to another of . different focal length. Not only is it often essential that this change be made in a minimum period of time, but also the projectionist must know that the newly installed lens will, without any adjustment, project a perfectly focused picture to the screen. This is all taken care of by the new fixed focus lens clamp, L, Fig. 448 and Fig. 449, and F, Fig. 450, which is locked rigidly to the lens barrel by means of screw N, Fig. 450. The lens is first mounted without ring L, and the picture is focused as sharply as possible upon the screen. Ring L is then slipped on over the front end of the lens barrel, and shoved back until it rests snugly against the front lens clamp as at L, Fig. 449, whereupon screw N is set up tightly, which locks the ring to the lens barrel in immovable position. Having properly locked the ring into place it is obvious that if the lens barrel be then pulled out and reinserted, if it be shoved in as far as ring L, Fig. 449, will permit it to go (until ring L comes into contact with lens clamp H, Fig. 449), it will occupy precisely the same position with regard to distance from the film that it did before it was taken out, hence the picture will be in sharp focus on the screen when it is again used. This being true, it follows that if we have as many rings as there are lenses to be used, and if a ring be installed on each lens as above set forth, then we may pull any one of them out and install any one of the others, with full confidence that the focus of the picture will be perfect when it is used.