Postby Maris » 06 Mar 2015, 04:55
A lot of complicated stuff happens when developer concentration changes.
A more dilute developer has a lower ph (depends of buffer capacity of the formulation) and is therefore less active.
The rate of diffusion of developer molecules into emulsion is faster at higher dilutions.
But fewer developer molecules available for mass transfer means the redox reaction between developer and exposed silver halide crystals is slower.
The rate of diffusion of development restrainers, usually liberated bromide ions, out of the emulsion is faster for more dilute mixtures hence less restrainer effect.
The effect of grain modifiers, usually silver solvents, is reduced at higher dilutions so some large (but fewer) grains grow more quickly: grainier negatives.
At high dilutions the relationship between developer molecules and silver halide crystals approaches stoichiometric and the developer can become exhausted before all available silver is reduced.
The preservative in a developer (usually sulphite) becomes less effective at high dilutions so more developer molecules get oxidised before they even get to the silver halide crystals.
The net effect is that more dilute developers work slower but the effect is non linear. Empirical testing (tedious) is the only sure way forward. In practice no one adjusts negative contrast by jogging developer dilution up or down.