Moving Picture World (Jan-Feb 1925)

January 31, 1925 MOVING PICTURE WORLD 507 idealism within the craft have been of untold benefit to our membership and the discussions you have aroused on many occasions have caused men to think; and when men think they act. The result has been a contribution to the Industry as a whole which is wholly creditable and for which you have Just reason to be proud. I trust that your further efforts in your chosen field will be productive of unbounded benefits to those whom you serve and my New Year wish to you is that your success may be unbounded and your personal welfare will have all the advantages that can possibly be bestowed by a Divine Providence. Personally I am always proud of your work, and to borrow from Robert Louis Stevenson, may I not say “Of what may a, man be proud if he is not proud of his friends?” Most cordially and sincerely yours, CANAVAN. The Lens Chart Two men whose envelope imprint declare them to be operators of a theatre, but whose letter is written on the stationery of a projectionist a local union and whose questions cause me to presume them to be motion picture projectionists, and not theatre operators, write thusly: Am writing with regard to the lens charts just lately received. Desire to use them, and believe that with your assistance we can get more light on the screen. The point is, I am not confident of my ability to apply the chart in the best way, and will advise as to just how far I have got, which may save me the trouble of sending lenses, condensers, etc., back to the factory. (Condensers are "lenses.” We differentiate by designating them as “collector” and "converging” lens, or condenser lenses, AND (or) projection lens. — Ed.) The conclusion I arrive at, as a major point, is that the two T distances (meaning the Y distance for each projector? — Ed.) must be the same, if we are to get 100 per cent, efficiency, and that in order that they be the same we must have certain size lenses (projection lenses. — Ed.) and condensers. Good Arrangement Here is what I would call a good optical arrangement, according' to the charts. Is it right — that is for the job where I expect to use it? I have selected first, as a basis upon which to work, a Y distance of 17 Inches, and propose securing lenses (Projection lenses. — Ed.) and condensers to fit this distance. My transverter delivers 70 amperes at 60 volts. My carbon combination is 3/8 inch negative and 7/8 inch positive. Carrying 70 amperes my carbon crater, which is approximately 29/64ths of an inch in diameter, would, I believe, require a 17-inch as the crater Y distance. (The crater distance is the distance crater to face of collector lens. There is no such thing as a “crater Y distance,’’ but a Y distance, which is from face of converging lens to film. — Ed.) The projection lenses are to be either 2 7/16 or 2 1/2 Inches at the smallest point, (meaning that will be their free diameter. — Ed.), which I believe would demand a 17 inch distance. I have selected, to go with the foregoing conditions, piano condensers sized 6% and 11%. (presumably meaning that you have selected a 61/2 inch focal length collector lens, and an 11% inch focal length converging lens. — Ed.) The meniscus may be better, but I cannot decide what combination to adopt, hence the piano. Also I propose securing a shutter shaft extension for Power’s machines (Power’s projectors.— Ed.), and placing the rotating shutter w'here the light rays apparently cross (at plane of aerial image. — Ed.), which, under the conditions named, would be' exactly nine inches from the face of the projection lens (presumably meaning the front surface of the element of the projection lens next the screen. — Ed.) In this position I doubtless could reduce the width of the shutter blades, which latter will be two-winged, made from cardboard. The lenses will be Gundlach, direct from the factory, and matched — size 6 3/4 inches. (I think you must mean that will be their equivalent focus. — Ed.) My picture is too small, but there is no new curtain (Meaning screen. — Ed.) in sight as yet. The throw (projection distance. — Ed.) is from the balcony down, and is approximately 140 feet. No Bluebook My good friend surely you have not got a Bluebook, if you have one you have not studied it to very good purpose. For goodness sake learn the correct nomenclature of your profession. I’m quite sure had you mentioned your projection room, it would have shown up in your letter as a “booth.” Your lenses might, from your nomenclature, be taken as 6% inches in diameter. You have a Transverter. Why call it a Transverter machine? Certainly it is a “machine,” just as your projectors are. Wake up, man. This is 1925 — NOT 1910, and things projectional have changed since that fourteen-year-off day when we had “machines,” “booths,” “coops,” “O’praters,” “throws,” etc. No, I’m NOT “roasting” you — just trying to get you to realize that projection is ADVANCING, and that the old. Silly, shop-worn and rather idiotic names of yesteryear are being dropped by all men who take pride in their PROFESSION. Evidently you have not yet quite grasped the underlying, basic principles upon which Mr. Griffith constructed the lens charts. They are designed to do either one of two things, viz: advise you as to just what you ought to have in the way of a condenser and projection lens free diameter, or tell you how to get the best possible results from what you already have. The basic thing is the crater distance (distance crater to face of corrector lens), because experience has taught us that under average conditions of lamphouse ventilation a crater of given diameter may be burned at a certain minimum distance from the collector lens without excessive lens breakage, AND this is of enormous importance, in light conservation, as you may see by referring to page 162 of your Bluebook, if you have one. Mr. Griffith built the lens charts based upon this possible minimum distance, and that is the first thing the user of the charts should consider. Your Problems You say you have a crater approximately 29/64ths of an inch in diameter. I have told you all, time, time and again to send impressions of two or three of your craters, made by pressing them on clean white paper, preferably with a blotter under it, the latter merely serving to insure a good, clean impression. I can then measure the exact diameter, and there will be no “approximately” to it. Examining chart A, we find by looking in the condenser combination vertical column, that such a crater (ordinary arc, which I presume you have, as you have said nothing about high intensity and have named an ordinary arc carbon combination) may be maintained at 3 3/16 inches crater distance without excessive lens breakage. This we find by consulting column Y, calls for a 12j4 inch Y distance, which we instaintly see is too long for a 6)4 E. F. projection lens, which we may assume to have about 5j4 inch working distance. 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