This section is from the book "The Fundamentals of Photography", by C. E. K. Mees,. Also available from Amazon: The Fundamentals Of Photography.
Fig. 58. Growth of Image During Development (to be desired)
Fig. 59. Growth of Image During Development (actual)
Old-time photographers used to take pride in the accuracy with which they could judge the progress of the development of negatives, and it was regarded as quite wrong when, in recent years, people insisted that negatives could be developed just as well by timing the development as by watching it, and that it was better for the negatives not to be watched.
The customary way of judging the progress of development in a negative is to hold it up to a lamp and look through it, but unless one has had a lot of experience he is very likely to be deceived because the apparent density of a negative held up to a light is very difficult to judge. The emulsion which has not been developed makes it appear stronger than it really is, and beginners almost always under-develop negatives if they try to judge when to stop development. If for some reason it is necessary to judge the progress of development by inspection (and this applies particularly to lantern slides), the best way is to turn the emulsion side to the light and look through from the back. This is much less misleading than if they are examined from the front.
There is no doubt, however, that the best method of judging development is simply to develop for a fixed time.
Films are best developed in a film tank, and the time of development, at a temperature of 65° for the tank developer, is 20 minutes. This time depends on the temperature. If the temperature is lower than 65° the time must be increased, and if it is higher than 65° the time must be reduced.
Instructions for development are furnished with each Kodak or Premo tank. It might be thought that if the film were over-exposed and so gave density easily it should be developed for a shorter time than if it had received less exposure, but this idea is quite wrong, because what is wanted in a negative is not correct density, which only affects the time of printing, but correct contrast, and the contrast is controlled by the time of development. An overexposed film will tend to have too little contrast, and if the development is lessened the contrast will be still further reduced and the negative will be flat. On the other hand, an under-exposed film tends to be too contrasty, and must not be forced in development or it may be unprintable, and so whatever the exposure, the best result will be obtained by the use of the normal time of development. Of course, the best negative can only be obtained by correct exposure as well as by correct development, and it is a mistake to think that we can correct errors in exposure by deviation from the correct time of development.