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LENSES AND MIRRORS 117
design, in which the glass elements become larger but the same speed, so far as possible, is employed.
Matching Lenses
(65) Should you wish to obtain a projection lens to match the one you have, it is best to send the lens to the manufacturer. (66) If you cannot do that, then send him the precise width of the projector aperture opening, measured in thousandths of an inch; also the exact distance from the lens center to the screen at the center of the vertical height of the screen image (with no film over aperture) ; also the exact projection angle. Do not rely on lens markings as they have an allowable tolerance of error which makes them only approximately reliable.
(67) One lens manufacturer has an excellent method of marking his product. When a lens is finished, the E. F. is very carefully measured. If the E. F. is exact in focal lengths it is marked on the barrel as 5-inch, 6l/2 -inch, etc. If the lens is a trifle more or less than the prescribed focal measurements, but is within plus or minus of one percent of the computed E. F., a plus or minus sign, as the case may be, is engraved in addition to the measurement. (68) All lenses marked without a plus or minus are considered as matched lenses, having even focus. The total difference in picture dimensions of a lens marked 6" — and one marked 6"+ would not in any case exceed 1 to 1.5 inches, which may be absorbed b) the black border.
Light Loss By Reflection
(69) Optical engineers estimate that the light re fleeted from clean, well polished glass surfaces is from 4 to 5 percent at each surface where the light meets the surface perpendicularly. It is higher where the light meets the surface at an angle, and increases as the angle becomes more acute. Some optical systems have as many as ten glass surfaces through which the light must pass — each one of which reflects its quota of light, (70) There are heavy losses at other points — at the spot and at the rotating shutter, the latter eliminating approxi