Home Movies (1944)

PACE 226 HOME MOVIES FOR IUNE Postwar Promises Coaled Cine Lenses Exhaustive Tests Being Made Known Anti-Reflection Coating Of All Methods B M ALTHOUGH you don't hear much about it these days, for war-time reasons, anti-reflection coatings on lens surfaces are becoming more and more an accepted, standard practice. For a while there were arguments advanced against it in some quarters; before the sound of controversy had died down, we were in the war, and the matter became largely academic for the duration, so far as civilians were concerned. However, it is not being lost sight of; cn the contrary. Government use of coated lenses is so extensive that the tens of thousands of photographers in the armed forces are coming to think of treated lens surfaces as normal and usual. It is probably safe to say that after the war all photographic lenses except the cheapest will have some form of anti-reflection coating. For that matter, to call these "antireflection coatings" somewhat minimizes their importance. Actually, their greatest value is in the reduction of scattered light, or flare. Even amateur knows that some light is lost in a lens — a tiny proportion absorbed by the glass, and a rather larger amount reflected toward the object. Now if this reflected light were simply lost, and that were the end of the matter, it would not be too serious. Unfortunately, the light reflected back from the second glass-air surface hits the first glass-air surface, and some of it is again reflected back toward the film. This happens at each surface, and the stray light keeps bouncing around the inside of the lens mount, between surfaces, and since most of these surfaces are curved, they scatter the stray reflected light in all directions. Thus the stray light which bounces back a second time doesn't return along its original path, but wanders in another direction. As a consequence, some of the light from the highlights is scattered into the shadow areas, flattening contrast, destroying delicate detail, and often working harm far beyond its relatively slight intensity. If a highlight has a brightness of 100 units, and two units of scattered light are added, the effect will be minor. But if a shadow has a brightness of one unit, and two units of scattered light are added, the effect will be serious. Furthermore, in the C2se of some lenses, the scattered light will not be at all uniformlv distributed ever the film, but will be concentrated in certain "flare" spots. All of this is largely counteracted by an anti-reflection coating. The coating is not 100' , effective, but it is so nearly so that flare drops to an inappreciable level. Some old lenses which are practically unusable because of flare can be turned into excellent objectives by suitable coating. As for the gain in speed, this will be from 30 to 40' < with an average lens. If the number of air-glass surfaces is very small, the gain will be less; with some lenses, higher. Consequently, when we have a more rational system of marking lens speeds than the F value, the speed markings on a lens would be altered after coating, to correspond to the increased light transmission. The cost of coating an average lens, under commercial conditions, runs from ten to twenty dollars, using the best methods. Probably, after the war, methods costing much less will be available. Some lenses, such as those with few airglass surfaces and not subject to seri ous flare, would scarcely justify the cost. Such things can become a fad, and many photographers have had lenses coated, simply because "it was the thing to do," which were little better after treatment than before. Judgment must be used in such a matter. When in doubt, ask the manufacturer; if that is impractical, write Home Movies and the editors will try to obtain the information for you. The same caution applies to the type of coating to be applied. There are coatings applied by evaporation in a vacuum, by vapor treatment, by dipping, by heat treatment, by tarnishing, by etching — and each has its uses and its strong points. The most efficient coatings are too fragile for external surfaces — the cheapest are not suitable for all types of glass — the most durable cannot be applied to cemented lenses, and so on. However, we hope to keep readers posted from time to time, as these things become available again for civilian use. Many movie amateurs have often been puzzled by references, in catalogs and elsewhere, to the terms "A Mounts" and "C Mounts" for 16mm. camera lenses. The Wollensak Optical Company explains difference between the two as follows: • Continued on Page 24} Delrv Contest Winners Announced When the DeVry Corporation went directly to the amateur movie maker for ideas for their new postwar cine cameras and projectors, they got exactly what they were looking for — sound, practical ideas for better equipment. The thousands of ideas submitted in the DeVry nation-wide contest, which was announced last November in Home Monies, have been analyzed and contestants contributing the soundest ideas have been awared prizes totaling Si 500 in value offered by the DeVry Corporation. Home Monies' editors are pleased to note that one of its consistent contributors, a demonstrated dyed-in-the-wool amateur movie fan. is among the winners of top awards. He is Robert C. Denny of Fresno, California, who won third prize for camera design. Other contest winners were George J. Fleim of Milwaukee. Wise, who was awarded first prize for the best cine camera design submitted in the contest. Fred A. Armster, Mt. Rainier, Md., won first prize for best projector design. Second prize for best camera design went to Douglas G. Sites, Havre DeGrace, Maryland. Second prize for best projector design was awarded to J. J. Mulkey, Fairbury, Nebr. Numerous prizes of war bonds were awarded for suggestions for mechanical refinements in cine camera mechanisms and projectors and it is notable that more than 98 ' , of the ideas were submitted by amateur movie makers, most of them readers of Home Movies. While details of all ideas submitted cannot be revealed until after victory is nv on and manufacture of civilian goods can be resumed, it can be said that they will have a very definite effect in improving the performance and quality of cine cameras and projectors offered the movie amateur by DeVry after the war. Wm. C. DeVry and the DeVry Corporation are to be complimented upon their sagacity in going directly to the experienced amateur for sound ideas for their post-war cine equipment.