@ 1) There are many well known restrainers, which can be grouped into two main classes:
- Inorganic restrainers, typically halides like bromides and iodides. These compounds shift the development reaction to the right and thereby increase the required developer activity level to develop silver halide grains.
- Organic restrainers. These are organic molecules, which form poorly soluble silver salts and thereby inhibit the development reaction. Like inorganic restrainers they increase the required developer activity level to develop silver halide grains. Common examples of such restrainers are Benzotriazole, Nitro-Benzimidazole or Phenyl-Mercaptotetrazole, but many are known in photographic literature. Their most important property is the solubility product of their respective silver salts - stronger restrainers form the more insoluble silver salt.
One important difference between different restrainers is also to which extent they get used up during development. Those restrainers forming more insoluble silver salts will replace the bromide anion in silver bromide emulsions and thereby get used up. This is especially relevant if one reuses developers. Color developers are designed such, that the bromide and iodide released during development together with the replenishment regime keeps the concentration of bromide and iodide unchanged between dev runs.
There are in depth descriptions of different restrainers in common photographic literature, some of which is freely available due to expiration of copyright.
@ 2)
If you look at an exposed emulsion, you need different levels of energy to develop different silver halide grains:
- The highest amount of energy is needed to develop unexposed grains. The result would be fog.
- Then there are weakly exposed grains. If they get developed, you get shadow detail. If film is very fogged from old age or poor storage, some of the unexposed grains are easier to develop than weakly exposed ones, this is where aged film loses speed, since you can no longer differentiate between weakly exposed and unexposed grains.
- Strongly exposed grains need comparatively little energy to get developed.
If restrainers raise the energy required to develop grains, and if the rest of the developer doesn't change, then increasing amounts of restrainer will at first eliminate fog, then eventually lower film speed, until it finally prevents development altogether. As you add restrainer and transition from stage 2 to 3, you lower film speed to the point, where you get increased contrast, so you will typically find lots of restrainer in high contrast developers.
Note, that some of these restrainers not only form poorly soluble silver salts, but at high concentrations form soluble complexes with silver. There are fixers based on concentrated solution of Ammonium Iodide! Therefore you have to be careful when adding excessive amounts of restrainer, at some point they will act as solvent which will increase developer activity.
@ 3)
As mentioned previously, as you add restrainer you at first eliminate fog, then you reduce film speed and increase contrast, then you eventually suppress development, and with many restrainers further increase will actually reignite the developer. Not every organic restrainer does this, and not ever restrainer is water soluble in amounts necessary to form these complexes.
If one wants to reduce contrast with restrainers, one can not just add it to the developer. Special restrainer release techniques are necessary to trigger release of restrainer in strongly developed areas. This will both increase sharpness through adjacency effects and reduce contrast:
- High iodide emulsions such as Delta 3200 release enough iodide during development that contrast is lowered and stronger pushes are possible without runaway contrast.
- Color negative film releases strong restrainers when special couplers get in contact with oxidized developer. Instead of forming dyes, these DIR couplers release compounds like Phenyl-Mercaptotetrazole to severely restrict development in these regions. This is what gives Ilford's XP2 its huge latitude and sharpness together with fine grain.
Fully acknowledging your request to limit this discussion to film, I still encourage you to do at least initial tests with othochromatic paper, since you can observe developer activity during the process in red light. You will save yourself from endless waste of material, if you get the hang of these restrainers before you start fine tuning film developers.