Amateur Photographer & Cinematographer (1937)

December 22nd, 1937 Ihi AMATEUR PHOTOGRAPHER p 6 CINEMATOGRAPHER 0 ^Deciders Questions *V ANSWERED General. — All communications for the Editor should be addressed : “ The Editor, The Amateur Photographer and Cinematographer , Dorset House, Stamford Street, Loudon, S.E.r,” and in every case. without exception, must give the name and address of the writer. Contributions. — The Editor is glad to consider original, up-to-date manuscripts on photographic subjects. All contributions must be typewritten, or in very legible handwriting on one side of the paper only. Letters or communications arising out of matters alreadv appearing in the paper are not paid for. The Editor disclaims legal responsibility for the safety of matter submitted to him, but he will endeavour to return rejected manuscripts, etc., when a stamped addressed envelope is enclosed. MSS. or prints that are not actually accompanied by a stamped wrapper or envelope can in no case be returned. Reproduction fees for prints are only paid by arrange ment beforehand. The sending of a print, without any condition stated, will be taken as permission to reproduce it without fee. Enquiries and Criticism. — Advice, Criticism and Information are freely given, but the following conditions should be read carefully before applying, as any communication which does not comply with the rules must be ignored. (1) See “ General ” above. (2) Every question and every print for criticism must be sent separately through the post, and must be accompanied by a separate stamped addressed envelope. No exception can be made in any case to this rule, except so far as enquiries or prints from Overseas are concerned. (3) Neither enquiries nor prints for criticism must be enclosed with competition prints. (4) On the back of each print sent for criticism, in addition to the name and address of the sender, must be the title (if any), and the criticism coupon from the current issue. (5) Enquiries should be clearly written, on one side of the paper only, and should be specific and not general. Such enquiries as “ How can I take interiors ? ” or “ Can you give me some hints on outdoor portraiture ? ” are too general to be dealt with in this section. (6) All envelopes should be distinctly marked “ Query ” or “ Criticism,” as the case may be. (7) Prints are sent for advice or criticism on the distinct understanding that by so doing per¬ mission is given for their reproduction without fee. We endeavour to deal promptly with enquiries, but cannot undertake to answer by return of post, nor can we give precedence to any enquiry. A selection of those replies is printed each week, but all questions are answered by post. Enquiries from abroad must contain a coupon also, but it need not be from the current number, and should be cut from the latest issue to reach the enquirer. Pinholes. The enclosed negative, developed by a commercial firm, has a number of clear specks on it. Can you tell me how these arose ? If I developed my films myself should I get as good results as by having them done for me ? A. S. R. A. (Cambridge.) The spots on your negative are what are known as pinholes, and are due to dust on the surface of the film during exposure. You must take every care to keep the inside of your camera quite free from dust. There is no reason why you should not very quickly learn to develop your own films quite as well as they are done by the average profes¬ sional firm, but for the best and most certain results we would strongly recom¬ mend you to invest in a tank for the purpose. Lamp for Enlarger. I have just built an enlarger, and would be obliged if you could tell me what type of electric lamp to use. Is there any made specially for the purpose with low emission of heat ? W. S. H. (Southampton.) It is usual to employ for enlargers ordinary household lamps of about 60 watts. The amount of heat generated depends only on the wattage of the lamp, amd not on its make or type. Hyperfocal Distance. I have an Ensign Selfix camera fitted with a 100-mm. Ensar lens of aperture f/4.5. Would you please tell me the hyperfocal length of this ? When focussed for this distance, from what distance to what distance will objects be in focus ? F. D. (Bristol.) If you refer to pages 12, 13 and 14 of The A.P.” Diary for 1938, now on sale at is. 6d., you will find full instructions for calculating both hyperfocal distances and depth of focus at any setting of the focussing scale. For a moderately high standard of definition — but not the highest — the hyperfocal distance of your lens would be roughly 70 ft., and when the scale is set to this objects from 35 ft. to infinity would be reasonably sharp. Photographing in a Mirror. Recently I took a photograph of a reflection in a mirror, carefully measuring the distance from camera to mirror and setting my focussing scale to correspond. Yet the picture is very badly out of focus. Can you tell me why ? M. V. S. (London.) You do not say whether your photo¬ graph included the frame of the mirror ; if it did, that frame should be sharply rendered, but the image you were trying to photograph certainly would not be. The reason for this is that the image does not lie on the surface of the mirror as though it were painted on the glass, but is behind it. You can prove this for yourself by holding a small mirror, in¬ clined at an angle, an inch or less from your eye. Though your eye could certainly not focus anything as close as that, you will still see a perfectly clear image of the objects reflected, proving that the image is much farther away than the surface of the mirror. Actually it is as far behind the glass as the object reflected is in front of it. In photo¬ graphing a reflection you must therefore measure the distance from camera to mirror, and the distance from mirror to object, add these two distances to¬ gether, and set your focussing scale to the total distance so obtained. Your picture should then be sharp, but the frame of the mirror will of course be blurred. Gum-Bichromate Printing. Can you tell me where I can get a handbook on the gum-bichromate process, and where I can buy the sensitised paper for it ? A. J. R. (Surrey.) We know of no book now in print dealing with the gum-bichromate process only, although the matter is dealt with in various textbooks, such as the " Dictionary of Photography.” You cannot buy the paper required, as all workers of this process prepare their own. Re-Silvering. Can you give me the name of a firm who would re-silver the mirror of my reflex ? Mine, new eighteen months ago, is already badly tarnished through the perishing of the lacquer. Would not a chromium-plated metal mirror serve equally well ? It would certainly last better. H. E. S. (Birmingham.) There are several firms who will resilver a mirror for you, and one of your local photographic dealers can get the work done. Alternatively, you could return your camera to the makers and let them carry out the work. Reflex mirrors are generally silvered on the surface so as to avoid double reflection, and are varnished afterwards. Such a mirror ought to last much longer than the time you mention. We know of no reason why chromium-plated metal mirrors should not be used, but we have never heard of this being done. Finding Focal Length. Will you please tell me how I can find the focal length of a lens ? S. V. (Weymouth.) You can get a fairly close approxi¬ mation to the focal length of a lens by focussing sharply on a very distant object, and measuring from the back of the lens to the screen in the case of a single lens, or from the diaphragm to the screen in the case of a compound lens. If your camera has extension enough, or if you can rig up some tem¬ porary arrangement, you can get a still closer approximation by focussing a small object so that it is reproduced exactly full size on the screen. The focal length will be almost exactly onequarter of the distance between object and focussing screen. The most exact method of all is to focus first the distant and then the near object as described ; the extra lens-to-screen distance re¬ quired for the near object, over and above that needed for the distant one, will be the exact focal length of the lens. Copyright in a Window. Recently I took a photograph of a memorial window in a church, erected by members of his family in memory of a soldier. This photo¬ graph having come out well, I should like to submit it for publication. If I did so, should 1 be breaking any regulations concerning copyrig ht ? H. K. (Sheffield.) We would strongly advise you not to publish the photograph you have taken without getting permission from the persons who had the window pro¬ duced for them. You should also get permission from the rector, or vicar, in charge, who will doubtless be able to put you in touch with the people who gave the window to the church. Safelight for Bromide Paper. I have read of bromide paper being developed by a yellow light instead of a proper dark-room lamp. If this is safe, can you tell me where I can get the necessary yellow filter ? I. B. (Bolton.) A yellow safelight is generally used for bromide work, and you will find it a great deal more comfortable to work by than the deep red light you have apparently been using. Any dealer can supply you with a suitable safelight, but you should make sure you let him know what kind of lamp you use. 32 715