Amateur Photographer & Cinematographer (1937)

April 28th, 1937 The AMATEUR PHOTOGRAPHER o 6 CINEMATOGRAPHER p V peed— and still more Speed 1 Hypersensitising Super-Speed Films Whatever camera he uses, every worker is bound to find some Coronation subjects just outside his range. Hypersensitising may add just the extra touch of speed that makes them possible. The following notes have been taken from an article by Harrison Fisk in “ American Photography,” and are supplemented by a quotation from “ The Professional Photographer.” EVEN with the ultra-speed f/1.5 lenses which are obtainable for some modern cameras, more speed is occasionally required. When the fastest film available is not fast enough under existing conditions, it may be “ hypersensitised,” usually by one of two methods. By such treatment of the film the extreme-speed lenses are made even more effective under the most adverse of conditions, and owners of slower lenses are enabled to equal the results of a faster lens when used with untreated film. The two bestknown methods available for the purpose are ammonia fuming and pre-fogging. Fuming with Ammonia. Ammonia fuming is probably the simpler and more certain of the two procedures. An increase of 100 to 400 per cent over normal film speed is claimed for this method. The materials needed are a film developing tank with reel, a piece of blotting-paper, and the strongest available ammonia. The piece of blotting-paper is trimmed so that it can be placed in the bottom of the developing tank and then is saturated with the ammonia. Any excess ammonia is allowed to drip from the paper, which is then put in place in the tank. In total darkness the unexposed film is wound on the developing reel, just as if it were ready to be developed. The reel is inserted in the tank, which al¬ ready contains the blotting-paper soaked with ammonia, and the lid replaced. The fumes of the ammonia should be allowed to affect the film for five minutes, at the end of which time it should be removed from the tank, in total darkness of course, and if it is to be used at once it may then be loaded into the camera. Keeping Fumed Film. Film treated in this manner will soon revert to the original speed which it possessed before treatment, and will have lost the effect of the ammonia within a day or two if it has not been exposed and developed in the meantime. The treated film should therefore be used and processed as soon as possible after contact with the ammonia. A treated film may, however, be preserved for a few days, if necessary, in a refrigerator, since the hypersensi¬ tised emulsion retains its speed for a longer period of time in cold air than at room temperature. In loading into the camera a film which has been kept in a refrigerator, the film should be allowed to warm up to room temperature before being placed in the camera in order to avoid possible con¬ densation of moisture on the film surface. A few other precautions should be noted : Strong ammonia must be handled carefully to prevent chemical burns — on no account must any be permitted to reach the eyes or throat ; soaking the blotter is best done in fresh air or near an open window to avoid the effect of the fumes, beneficial to the film, but uncomfortable to the human race ; and, of course, only the fumes should reach the film, no liquid ammonia. Pre-fogging is rather a tricky and less reliable way of increasing film speed, although some workers are quite success¬ ful at it. The practice of pre-fogging is based on the theory that there is a certain inertia in photographic emul¬ sions which must be overcome before any image is formed on the film. If, before use, the film can be given just enough exposure to remove this threshold of inertia, the emulsion will have a head start, and any subsequent exposure will be spared the exhausting labour of having to cross this threshold before it can form an image. How it is Done. The practice in pre-fogging is to turn the lens towards a blank, evenly illu¬ minated space, such as a plain white wall, or large sheet of white paper, and give a very short exposure. This is where the difficulty in pre-fogging is found, for it is not an easy thing to judge such an exposure so as to give exactly the effect desired. In ammonia fuming one can recommend a fiveminute exposure to the effect of the fumes, but in pre-fogging one cannot advise the photographer to " aim the lens at a blank wall and give an exposure of 1 /25th of a second at f/22,” unless the adviser knows just what colour the wall will happen to be and just how much illumination will be used, and unless he has considerable experience on which to base his judgment. Only personal experiments with a blank white space, given illumination, and a photo-electric exposure meter would give the worker a definite idea of just what exposure would yield the desired result. To give him some start¬ ing point from which to set forth on his investigations, it is suggested that he take a reading of the illumination of the blank space and then give the exposure which the meter records for the absolute minimum at that reading. By experi¬ 428 menting with similar exposures he should soon find one giving satisfactory results. The worker who possesses no dark¬ room or developing tank may be forced to resort to this method of hyper¬ sensitising his film, but the owner of such equipment will get uniform results with greater certainty by using ammonia. Mercury for Speed. A more recent method of dry hyper¬ sensitising has recently been published in the Journal of the Society of Motion Picture Engineers (America) and referred to by the American Professional Photo¬ grapher, from which we quote. The new method, which is the result of experiments carried out at the Agfa Ansco Research Laboratories, is extremely simple and surprisingly effective, giving from 50 per cent to 150 per cent increase in emulsion sensitivity. To effect the hypersensitising, wrapped or unwrapped film is merely placed in a sealed container with a small amount of liquid mercury (0.5 gram) or silver amalgam containing a high percentage of mercury. The film is allowed to stand at room temperature from 36 hours for loose or unwrapped material, to about a week for wrapped or tightly spooled film. The sensitising action of the mercury vapour is reported to be slow, but so even that no streaks or uneven markings occur. Actual con¬ tact with the film is prevented by placing the mercury in an open glass or metal container or by wrapping the amalgam loosely in porous blotting-paper. Treat after Exposure. A surprising fact revealed by the experiments is the unusual behaviour of the latent image when treated with mercury vapour. It was found that film showed a markedly greater increase in sensitivity when hypersensitised after exposure instead of before exposure. Another interesting feature of the method is that it shows no apparent effect on the gradation or the grain size of the photo¬ graphic material. The increase of sensitivity is general throughout the range of wavelength of light to which the film was originally sensitive, and the stability of the film is not permanently affected, although the increase in speed is gradually lost over a period of four weeks of ageing. By a second application of mercury vapour the hypersensitising can be renewed in a film that has recovered from previous treatment.” 35