Film and Developers
Inherent film characteristics can be altered by development. The following are some general guides.
While film speed cannot be directly controlled by development, it does play a part. Exposure controls density, and development controls contrast. A common practice is to expose a film at a lower speed than recommended. This helps with shadow detail, as there is longer exposure, while keeping contrast close to normal.
Another common technique is to "push" film. This means exposing it at a higher rating, and then compensating by development to get usable images. Usually it is done for low-light situations. An example is using Ilford's HP5+, which is normally rated at 400/27. In their data sheet, Ilford gives development times for pushing the film up to 3200/36 (3 stops). It does require particular developers, however, and the results will be less than optimal. That being said, for the most part pushing film that hard is an effort to get that usable image, not an attempt to get optimal.
Developers can both enhance grain and help to minimize it. Using a diluted developer, extending the time to compensate, forms the clumps in the silver slower. This makes for less apparent grain. Using the same developer at a higher concentration does the opposite; makes the grains clump faster and less evenly.
The developer temperature also affects grain: warmer = more. Films are generally designed to be developed at 68°F/20°C, but can be processed at other temperatures. Again using HP5+ as an example, using Kodak D-76 at a 1:1 dilution, Ilford recommends 11 minutes at 68°F/20°C. at 75°F/24°C, the time drops to about 6:45. Faster development, but more grain. One also risks fogging, or density in clear areas. Time alone will not necessarily affect grain, but usually shorter times produce more grain.
Over-agitation accelerates the development process, and produces greater grain. Agitation should be done smoothly and evenly. Just enough, not too much. (Too little wil result in uneven development).
A fine-grain developer such as Kodak D-23 (Darkroom Cookbook, p. 212), may also be used, but usually at the loss of film speed.
Contrast is perhaps the most important film characteristic affected by development. With contrast control playing a critical role through the entire image-making process, control at the negative becomes very important. Getting the contrast correct, how you want it, in the negative goes a long ways towards making the print.
Exposure controls density, and development controls contrast. This statement means a lot. First, get the information you need, particularly the shadow density (through careful metering and placement, then the subsequent exposure). If the shadow detail (density) is not on the negative, recorded on the film, it cannot be produced by development. Then use the development step in processing to control the highlight density.
The shadow areas, being thin, will be developed fully early in the development stage. The highlight areas, having greater density, will take longer. Simply, the longer you develop the film, the greater the density in the highlights, producing greater contrast, up to the point that the highlights are maxed out (maximum density is reached. Altering development from normal controls this density: shorter time, less density, more time, more density.
This effect is unique to each film/developer combination, and is plotted on a graph called a characteristic curve, with graphs log exposure versus development time. The steeper the curve, the greater the contrast.
While using development to control contrast in this manner works best with sheet film, as you can treat each exposure individually (the Zone System), it can be applied to roll film, at least in general. If a roll is taken under generally similar lighting conditions, contrast can be added or reduced to produce more ideal negatives.