Amateur Photographer & Cinematographer (1937)

December 22nd, 1937 I Thi AMATEUR PHOTOGRAPHER Ip 6 CINEMATOGRAPHER 0 when, and only when, all is in readiness, the show may be announced as ready to begin . The operator should, to ensure that the show shall go off without a hitch, if possible, take all possible precautions beforehand. He should, of course, have a torch handy, and a spare projector lamp, just in case. The films should be stacked in the order in which they are to be projected, the first reel already loaded and threaded in the machine. Another place should be ready to stack the finished reels, so that there shall be no confusion when the show is on. With regard to incidental music, this should be added if possible, and, in addition to the light music that would naturally be used for a programme of this nature, it would be a good idea to work in some Christmassy music, per¬ haps in an interval. In this way it will be possible to make the Christmas show the great success it ought to be. Letters to the Editor J The Editor is not responsible for the opinions of his correspondents. TESTING A LENS. Sir, — I have seen it stated in " The A.P.” and elsewhere that the circle of confusion of a photographic lens varies from i/iooth in. to a limit of about i/5ooth in. in the finest examples. As this seemed to mean that enlargements of over five diameters would show fuzziness, even with the finest optical equipment, and as many miniature negatives are enlarged very much more than this, a test was indicated. The test object was a diagram in the form of a wheel of radiating black-and-white spokes photographed on Kodak S.S. Pan. film in a Baby Ikonta, with f/3.5 Zeiss Novar at full aperture. Microscopical examination of the developed negative showed that the lines lost their identity and merged into a uniform grey tone when their separation was about i/500th in., and that much of the diffusion was due to grain. Had a thin-emulsioned film been used in a precision minia¬ ture I am sure that 1/ 1,000th in. would have been resolved. My Zeiss catalogue states that the f/3.5 Tessar is superior to the Novar by £2 4s. and I should expect from it an even higher standard of definition. Apparently the claims made for miniature lenses are very modest. — Yours, etc., OWEN TUDOR. (Johannesburg.) PERFORMANCE OF CHEAP CAMERAS. Sir, — Mr. Allinson writes from Switzerland to advise me to visit Messrs. R. F. Hunter Ltd. and see some large prints from Rolleiflex negatives. I am not easily able to do so, but have seen wonderful prints from these cameras before, but not taken at full aperture. I am a perfectly ordinary amateur who does general work. I am attracted by a large lens and coupled range-finder or other device for enabling the lens to be used at full aperture. I want at times to take snaps inside theatres, sports meetings, and the like, and as I do not specialise in these I want a camera larger than cine size. I have tried out so very many and not finding a good one turned me towards calculating the tolerance, and this showed me how very exact the construc¬ tion has to be and explained why none of them are reliable. As I am not arguing for the love of it, I ask any dealer who wants to make a sale to advise me if he has a camera in stock of the twin-lens or coupled range-finder type (the latter pre¬ ferred), taking 3JX2J or 2J square pictures, which he can demonstrate can be fixed on a tripod or table and a film exposed preferably on some printing, as follows : The whole spool to be exposed on at least four varieties of distance at an aperture of not less than f/3.5, and all negatives without exception are to show sharp prints when enlarged four diameters. I should much like to buy such a camera and have given up trying to find one. — Yours, etc., R. E. DICKINSON. FRONT-CELL FOCUSSING. Sir,— The following experiments I recently made with lenses focussing by the front cell method may be of interest to other amateurs. Being dissatisfied with the definition given by the lens in my camera — an inexpensive one of aperture f/2.9 — I resolved to test it. This was done by fixing a ground glass in the back, setting a 7 X magnifier so that it was exactly focussed on the ground glass surface, and examining through the magnifier the image of a page of print of different sizes set up at varying distances from the lens. The front cell focussing scale was found to be correct at 5 ft. at full aperture, but on stopping down to f/4, f/5.6, f/8 and f/11, it was found that to get the clearest image possible the front cell had to be screwed out a little farther for each reduction of the aperture, the position for f/11 being the focussing mark for about 3 ft. 6 in. The same thing applied, to a lessening degree, for distances of 7 ft., 10 ft. and 13 ft., becoming negligible at about 20 ft. ; at 10 ft., f/ 1 1, the clearest image being obtained at the 8-ft. setting. Bemoaning my stupidity in buying a camera with such a cheap lens, I hied me to Town and purchased another with one of the best lenses obtainable, of aperture f/4. 5, and, out of curiosity, I put it to a similar test, with exactly the same result, to a lesser degree. (Focussing in this second case was also by screwing out the front cell.) I therefore came to the conclusion that screwing out the front cell of a lens for focussing caused increasing spherical aberration, and the following test, 1 think, proves this. The lens was focussed on the same page of print at about 5 ft., at the full aperture, and the clearness of the image noted (print below a certain size was quite unreadable) ; the front cell was then screwed right in to the infinity position (its proper position for perfect correction), and the ground glass, with the magnifier focussed on it, was gently moved back until the image was formed on its surface. The difference was truly remarkable, print of the most minute size could clearly be read, and only the roughness of the groundglass made a limit to the definition of the image. Thus, apparently even with the best lenses, helical focussing cannot give pin-point definition at distances below about 15 ft. ; only radial focussing, or systems in which the whole lens screws in and out avoid causing aberration (this refers, of course, to apertures above f/8). Trusting that you will find room to reproduce this rather lengthy letter. — Yours, etc., T. D. CROOK. REVERSE SPOOLING. Sir, — A recent correspondent comments on the advisability of British camera manufacturers incorporating the idea of “ reverse spooling ” or a second winding spool to enable a half-finished roll film to be removed for the use of plates, etc., before the idea comes over from Germany. I should think the British manufacturers must have heard of, and seen, it years ago, as I have a combined J-pl. plate and rollfilm camera fitted with two winding spools which I bought as far back as 1909, and am still using it. — Yours, etc., CHAS. W. K. DALTON. SIMPLE CAMERA DESIGN. Sir, — Those of your readers who are old enough to have done hand camera work some thirty-five years ago will prob¬ ably remember the “ Xit ” cameras made by J. F. Shew & Co. These instruments were of such remarkably compact, simple and efficient design that I think they might well be resuscitated in a more modern guise. The “ Xit ” camera had the great merit of a really good rising front, and this without any added complication. The lens was focussed (as a whole) by a focussing flange, fitted with a distance scale. The camera was, so far as I remember, meant for use with plates in dark slides, the roll film of to-day being at that time very much in its infancy ; the design, however, readily lends itself to present-day film practice. An instrument of similar design, but using modern light alloy die-castings and stampings, and made to take the popular 2 J-in. square picture, would permit of full advantage being taken of the rising front for architectural work. The folding film cameras of to-day suffer from a multiplicity of pin-hinged joints, folding struts, etc., all necessitating extreme care in manufacture and assembly if true coincidence of the lens focal-plane and film is to be attained. All these 712 29