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68 MOTION PICTURE HANDBOOK pie, though a dense, dark mystery to very many operators. Multiply the distance from lens to curtain, in feet, by width of picture opening (aperture opening in motion pictures and slide mat in stereo), in inches. Divide this result by desired width of picture, in feet, and result will be equivalent focus of required lens, in inches. Example: What lens will project a is-foot motion picture at 60 feet? Answer: 60 x iO (aperture plate is it inch wide) equals 56j4. which divided by 15 equals 3}i, so we shall want a 3^-inch lens. Example: What lens will project a stereo pic- ture 15 feet wide at 60 feet? Answer: 60x3 (width of stand- ard slide mat) equalb 180, which divided by 15 equals 12, so we shall want a 12-inch stereo lens. This will not be abso- lutely accurate, since aperture plates are about 15-16 of an inch wide and mats usually exceed 3 inches by about l-i6th inch. Moreover, the cheap lenses are not ground absolutely accu- rate and there will be some variation there, too. But the rule answers for all practical purposes. The same result is at- tained by dividing the distance by the size of the picture and multiplying by the opening. In using this rule, don't guess at distances and expect accurate results. Measure from curtain to lens with a tape line, no matter if someone has "told" you it is a certain distance. Reject any odd inches in the measurement below six and add a foot if the fraction be more than six inches. If throw is more than fifty feet, meas- ure the aperture opening and multiply by its exact width to get accurate results, as on long throw the least variation of aperture will mean considerable on the curtain. Before starting the day's run remove the projection lens and make sure there is no oil on its back lens. If there is, you cannot get a clear picture. When using a lens-tube, the position of the picture may often be shifted on the curtain as much as a foot, up, down or sidewise, simply by turning the tube around in the jacket. About once a month take the pro- jection lens all apart and clean the lenses thoroughly by washing with wood alcohol and polishing carefully with a soft dry cloth or chamois. Because the projection lens looks perfectly clean and clear when you look through it is no evidence that it is so. Be very careful, however, to get the lens back just as it was, or you will have trouble galore. In