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FOR MANAGERS AND OPERATORS 121
it is very simple, and the key to this particular problem is "back focus."
In matching up a projector lens system, first, using the well-known formula for finding the equivalent focus of the lens required to project the size picture you want at the distance your condition calls for, determine the E. F. of the lens you want, procure it, mount it in the machine, and, using any condenser, project a picture, and very carefully adjust the objective until the picture on the scree'n is in sharp focus. Having done this, stick a rule through the aperture and, with its end against the lens, measure the exact distance of the rear surface of the rear combination of the objective from the film track surface on the aperture.
This measurement will be the BACK FOCUS at which your lens will work, and it is this measurement and not the equivalent focus, which must be used in matching up the lens system. The E. F. has absolutely no value whatever except to enable the operator to select the proper lens to project the size picture he wants at the given distance.
At this point we reach an item of much importance, concerning w/hich positive data cannot as yet be given, viz.: The selection of an objective lens of the right diameter to fit local conditions. Excess in diameter is undesirable, in that it is likely to set up trouble in the shape of travel ghost. Insufficient diameter, on the other hand, means loss of light, and loss of light is expensive. On the whole, it is much better, I believe, to get a lens of too large than too small diameter, because it is an easy matter to stop down the large lens to just the size needed, whereas the small diameter cannot possibly be made larger.
On the whole, I think the best recommendation we can make at present is that the E. F. of the required lens be found, and that a lens be ordered having a diameter equal to one-half its E. F., up to 4^ inches E. F., the diameter beyond that focal length to remain fixed at 2% inches, up to 7 inches E. F., beyond which it might possibly be increased to 2^ inches with advantage. When the lens is received, place it in the machine and focus the picture sharply on the screen, then measure the back focus, as already directed, and remove the lens. Now place a sheet of white paper inside the mechanism in the exact position occupied by the back surface of the lens, supporting it in any convenient way, without Jiaving changed the position of the lamp with relation to the condenser or of the lamphouse with relation to the aperture, strike an arc, and measure