International projectionist (Jan-Dec 1949)

'MATCHED' PROJECTION OPTICS (Continued from page 9) jection equipment are completed, specific recommendations should be issued. Until such recommendations are made, projectionists and lens manufacturers will be all but powerless to institute further improvements in the optical phase of projection. Why not employ the exact matching speeds instead of arbitrarily assigned optimum speeds which, at best, fall below ideal conditions? The practical difficulties of manufacturing and using extremely large lenses are serious, almost insuperable obstacles. Lens designers would find their skills taxed to maintain the necessary flatness of field and depth of focus. A host of troubles would descend upon the projection room, too, for the lens mount assemblies of many projectors would be too small to accommodate the barrels oi the largest lenses. Nevertheless, il is entirely possible that high-quality 77:1.5 lenses could be produced successfully in the shorter focal lengths, and these could be used in most theatres with gratifying screen results. Regardless of possible future innovations, the principles of matched optics plainly indicate the desirability — yes, the necessity — of using nothing less than the most rapid high-grade lenses available at the present time. The soundness of this recommendation cannot be disputed, yet thousands of theatres are still plodding along with outmoded low-speed lenses. Vignetting Effect in Lenses Optical experts have long been concerned with the so-called vignetting effect of projection lenses. This is a pronounced drop in screen illumination at the edges of the picture, and is especially prominent when the older, low-speed lenses are used. When the lamp is known to illuminate the aperture evenly, the cause of the vignetting effect must be sought in the design and dimensions of the projection lens. In most cases the nature of the light beam pouring through the aperture is a contributing factor. The central portion New 16-mm Film Source Directory Devotees of 16-mm films will be interested in the new Directory of Film Sources which lists hundreds of individual basic sources of 16-mm films divided into five basic classifications: general interest, special interest, educational, religious, and entertainment. Copies are available at 25 cents each from Radiant Screen Co., Chicago. BUY U. S. SAVINGS BONDS of the lens "looks" through the aperture and "sees" the arc lamp mirror or condenser beyond as a large disc of intense light. Ordinarily, however, the blazing disc does not appear quite as large as the condensing element to the "eye" of the lens, hence the bright disc is surrounded by a comparatively dark ring. Now, the edges of the projection lens "see" the opposite edges of the condensing element through the aperture. To the edges of the lens, however, the edges of the mirror or condenser are not dark at all but appear intensely bright! This shift of the magnified crater image with a shift of viewing point (on the surface of the lens) is due in part to aberrations of the curved "figure" of the mirror or condenser. (These aberrations also help produce the "bend," or bottleneck form, of the light beam from a projection lamp.) When a film is projected, the center of the projection lens "sees" a film picture which is very bright in the middle but dim at the edges. The edges of the lens, on the other hand, enjoy a rather different view of the picture. To them the overall illumination is less than it is to the center of the lens, but the edges of the film picture are the brightest parts of all. The several regions of the lens pass their "views" of the film picture to the screen where a highly magnified composite image is formed. The central portion of the screen image comes largely from the central portion of the lens, and the edges of the screen image come PRESTON E. SMITH— Owner of State, Tech, Plains and 5 Point Drive-In Theatres, Lubbock, Texas — declares: "RCA Service has been and will continue to be an intimate part of my operation." To get the benefits of RCA Service —write: RCA SERVICE COMPANY, INC., Radio Corporation of America, Camden, New Jersey. "HOW IT . WORKS' OLD STOP "Wondering About" Television MINE OF PRACTICAL FACTS By JOHN F. RIDER Television is in the eye of the public and in the minds of everyone associated with the motion picture industry. Here Are The Facts on: 1. How TV Pictures are Produced and Sent 2. What is in the TV Receiver 3. Installation and Orientation of TV Antennas 4. Recognition of Troubles in TV Receivers This book is written in down-to-earth language. You don't have to be an engineer to understand it. The entire book carries the practical along with the theoretical. 203 Pages Illustrated $2.70 Send Coupon Below TODAY! ............. ORDER WITH THIS COUPON--- International Projectionist 19 West 44 St., New York 18, N. Y. Enclosed find $2.70 for Television "How It Works" Name :..... Address City Zone State . INTERNATIONAL PROJECTIONIST March 1949 33