Probably the main differentiation in developers is between those that tend toward high definition (high acutance), and those that tend toward fine grain. Usually the fine grain developers contain sodium sulfite (or other substance) that has a slight solvent action on the silver grains that makes it less noticeable.(posted 8274 days ago)The characteristics of a particular developer can change by using different dilutions. For example, undiluted D-76 and Xtol are usually considered fine grain developers, but when diluted 1:3 the solvent action on the grain is much less pronounced (the percentage of sodium sulfite is less) and they produce fairly sharp negatives. Very high dilutions also can sometimes have the effect of increasing “edge sharpness” between areas of light and dark in the negative.
Obviously, the size negative you use (and how much enlargement is required) has a big impact on how much apparent grain you can live with. Ansel Adams had a definite preference for high resolution developers, but he usually used large format cameras (often 8x10) where grain is not a problem.
Rodinol is a high acutance developer (without any sodium sulfite). Used in low dilutions such as 1:25 (it is packaged as a very concentrated liquid), it is fairly high in alkalinity, which can increase the apparent grain (sometimes called grain clumping). Used in dilutions of 1:75 or 1:100, the alkalinity of Rodinol is reduced, and the developer becomes more neutral with regard to its effect on grain, while maintaining its very high acutance.
Diluting a developer can have other effects on the negative, such as contrast control. Usually, the more dilute the developer, the lower the overall contrast. Highly dilute developers are sometimes called “compensating” because of their ability to compensate for high contrast scenes or high contrast films. Since slow speed films (ISO 50 or less) tend to have more inherent contrast than higher speed films, many people tend to use a more compensating (diluted) developer with these films.
There are other special purpose developers (with or without sodium sulfite). Pyro is a “staining” developer, whereby a stain is deposited on the negative to improve the printability of highlight detail. Xtol is marketing by Kodak as a more environmentally friendly developer because of its main developer component being ascorbic acid. It is also supposedly easier to mix at room temperature, but some problems have been reported.
Then, there are developers which have the ability to have a greater than normal impact on film speed and/or contrast control (expansion or contraction using the Zone System). The effect of a developer on these factors often depends on the film being used. For example, I have found that Xtol gives a higher film speed (especially with T- Grain films) than most other developers.
Some people have a preference for developers that are packaged as a highly concentrated liquid solution, rather than a powder that has to be mixed and stored in a larger bottle as a “stock solution.” Once a developer is mixed into a stock solution (usually 1 or more liters), its keeping properties are decreased, especially for people who develop infrequently. But unmixed powder has a very long shelf life, which has its own obvious benefits. Rodinol is supplied as a very concentrated liquid and has legendary keeping properties. In the relatively small bottle of a developer supplied as a highly concentrated liquid, the air can conveniently be removed by placing marbles in the bottle as the developer is used up, something that is more difficult with stock solutions of 1 liter or more.