Home Movies (1944)

PAGE 356 I've dot A Problem!" HAVE you a perplexing problem in photography, editing, titling, or processing of home movies? Then tell it to the editors. This "problem untangling" service is free to every reader of Home Movies. Where answer by mail is desired, enclose stamped addressed envelope with your letter. Q: In answering Mr. Fail's query in 'Information Please," April, 1944 issue, you suggested setting up an "aerial focus system" to aid in re-calibrating a lens adaptation where camera permits focusing on film. Will you kindly explain the meaning of "aerial focus?" A: The subject of aerial focus is not too complicated and is something that will be of interest to every experimentally-minded cinebug with a smattering of learning in optics. When an image is examined on a very small ground glass, it is difficult to determine exactly where the point of best focus lies. If the image is viewed with a magnifying lens, grain of the ground glass is also magnified and no advantage is gained. However, since the camera lens plus the magnifying lens can be used to form a telescope, the ground glass can be eliminated entirely in the process. The only catch is that these two lenses have several different positions at which they will form a sharp image. So one more thing is needed — a device which will insure that the "telescope" image will be sharp only when the image is being formed exactly in the plane of the film. Suppose that crossed hairs are placed exactly in the film plane and the magnifier carefully focused so that the hairs are brought in sharp focus. In this position, the image through the camera lens will only be in focus when the camera lens is forming the "aerial image" precisely in the same plane as the crossed hairs, i. e., the film plane. So much for theory. In actual practice, all that is necessary is to make a small cross in the middle of the ground glass with a finely-pointed and reasonably hard lead pencil. Then cover this with a small piece of microscope cover glass, cementing it to the ground glass with a drop of Canada balsam applied over the penciled cross. This will make the glass completely transparent at this point, and all trace of the rough "ground" surface will disappear, leaving the cross to remain clear. This, plus the use of any good magnifier or reading glass is all that is needed. Surprisingly enough, the most accurate result is not obtained by noting when the image is sharp, since even such a precise arrangement presents some depth of focus. Best results will follow by setting up the device, focusing on an object, and looking through the magnifier. Keep the image of the cross absolutely sharp so that adaptation of the eyes doesn't deceive. Now move the head slightly from side to side. When the point of sharp focus is sighted, the image will not shift relative to the cross when thus observed. If it does move, then the focus is not sharp. Another convenient method is to fit snugly into front of the camera lens, a cardboard disc into which is punched two small holes about Ys inch in diameter near the left and right edges of the lens. Sighting through the lens as before, a double image will be seen as in a range finder. As the camera lens is moved back and forth in focusing, a point will be found at which the two images merge into one, indicating the point of sharp focus. Q: I am planning to adapt a 1 inch lens, originally used on a 16mm. camera, to my 8mm. camera. Will this adaptation cause any change in the f stops of the lens?— C. S., Lynchburg, Va. A: No, the stop values remain the same if the lens is mounted on the camera with the same relation to the film plane as when used on the i6mm. camera. Where such a lens is mounted in an extension tube, setting it farther ahead, thereby increasing its focal length, then the f values would be changed. This is further explained in an article beginning on page 146 of the May, 1943, issue. WAR FILMS ► Not everyone has opportunity to see all the newsreels showing latest invasion pictures. But every home movie enthusiast can see these pictures in the comfort of his parlor. Your photo dealer can supply Castle and Official Films releases in 16mm. sound of thrilling war action; he can rent you a sound projector, too, for screening the films. Get a party together for tomorrow night and show your guests thrilling sound movies. HOME MOVIES FOR SEPTEMBER Q: The last roll of film I shot is badly under-exposed. Most of the scenes, quite important to me, cannot be re-filmed. Is there any way in which this film can be reprocessed to lighten the image? — C. A. S., Beaver Dam, Wise. A: Your film can be lightened to a certain extent by treating it in a "reducing" bath composed of the following formula: Solution A Water 1 oz. Potassium Ferricyanide 15 gr. Solution B Water 32 ozs. Sodium Thiosulfate (Hypo)-. 1 oz. When thoroughly compounded, add solution A to solution B and immerse the film immediately. When film has been lightened to the desired density, remove from solution, wash thoroughly in running water and dry. Two precautions: the solutions decompose rapidly when mixed together, so must be used at once ; also, film to be treated should first be soaked in water a few minutes to soften the emulsion and make it more readily receptive to the reducing bath. Q: When winding back film in the darkroom, what color saielights can be used with various films? — I. F., Detroit, Mich. A: For positive film use a red, yellow, orange or green safelight. For ortho and semi-ortho films, use a red safelight. For panchromatic films, use the regulation green panchromatic safelight. Kodachrome and other color films cannot be exposed to safelights of any color and must be handled in the darkroom in absolute darkness. Safelights, to be truly "safe," should be kept at a maximum distance from the film and should never be too bright. A 10 or 15 watt bulb is usually recommended for safelights of all types. Q: Instructions on title making frequently state, in regard to placing title card in front of camera, "to measure distance from lens to title." Which lens is meant — the camera lens or the auxiliarylens?— A. W. D., Comptche, Calif. A: When an auxiliary lens is employed in filming titles, the distance from title card to lens is calculated from title card to position of the iris diaphragm of the camera lens. Position of iris diaphragm may be determined from the adjustable ring on lens that regulates opening and closing of aperture. This is usually located about the center of the lens. Thus, if you are using a 5 diopter lens which calls for an 8 -inch focusing distance, set title card 8 inches forward from iris adjustment ring on camera lens. Q: In shooting trick titles, how can I mount my camera upside down on my titler and still be sure lens is accurately centered on title? — F. B,, Santa Cruz, Calif. • Continued on Page }SS