This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Blank Spaces In The Developed Roll Film. Should you observe spaces in the film that are entirely blank, you have either very much undertimed that particular exposure or else your shutter did not work and no exposure was given. Upon developing further, should an exposure, which at the beginning appeared perfectly blank, commence to build up a trifle, cut out this film and place it in a tray of clear water. Let it remain there until you have completed the development of the film; then, take this under-exposed section of the film and place it in fresh developer. It should immediately commence to build up in density and in detail, although, in the majority of cases of under-exposure, the film will gain but little strength and it will be necessary to again place the film back into clear water, where it should remain undisturbed for five minutes or so. Repeat this operation from the clear water to the developer a number of times, allowing the film to remain in each for a period of four or five minutes. By this means you will obtain all the strength and detail possible in an under-exposed film. While in the developer remember the film should be kept in continual motion. This will assist the development and also prevent spots; but, while in the water the film should not be disturbed.
Black Sections In The Roll Film. After placing the roll film in the developer, should certain exposures flash up black all over, it is a sign of fog or extreme over-exposure. In that case, nothing can be done to save it and all attention should be given to the remaining exposures.
Securing Uniform Exposures On Roll Film. Those using roll film should make an effort to secure exposures as uniform as possible, as that will greatly facilitate work in the dark-room, and will also produce superior results. Time exposures and snapshots, to be successful, should not be made on the same roll of film. If they are, carefully judge the time necessary, so that long exposures in subdued or weak light will not have a greater effect on the film than the snap-shot in bright sunlight. Judge your light before making the exposure. If extremely strong, or should you be making snow pictures, with bright sunlight shining upon the snow, stop your lens down to f/16 or f/32 and use the instantaneous shutter or the 1-50 of a second. Then, when you come to make an exposure, on the same roll of film, of a dark object in the shade, use the lens practically wide open and give what you think the proper exposure; i. e, a corresponding exposure according to the light, which would be about one-half second. If you wish to make an interior of a room, not very well lighted, stop down to secure the desired depth of sharp focus. If you stop down to f/64 it may be necessary to give a minute's exposure to get a fully timed negative.
240. It is difficult to give absolutely correct directions for the making of the exposure, as light conditions are so different in almost every case, that it is necessary to experiment and to exercise considerable judgment.
Difficulties With Cut Films, Film Pack, Etc. The difficulties we have mentioned in the development of a roll film will apply equally to all cut films. Cut films should be developed face side down, and care must be exercised not to rub the edges of the film, which will loosen the emulsion from the celluloid support; nor must the edges of the film be allowed to come in contact with the emulsion side of any other film, as the result would be scratches and torn sections upon that film.
Softening Of The Film. In warm weather there is always a tendency for the emulsion to become soft, which will result in blisters and excessive frilling around the edges. The temperature of the developer should never be over 65o Fahr. in summer, and it is very advisable to use an acid fixing bath, which will harden the emulsion on the film and do away with any possibility of its softening when placed in the final wash water.