Why does a monobath work well for films that usually have significantly different process times?

Question

I've recently tried a commercial monobath, after seeing information about monobaths in general and this specific product on YouTube. I'm floored at the quality of the negatives. I've used a homebrew monobath once or twice before, but the results were only "okay". This time, my negatives look as good as I've have expected from HC-110 Dilution G or H (softer grain than Rodinal or Dektol).

What baffles me, however, is that I can develop different films, say same brand ISO 100 and ISO 400, in the same tank, for the same process time, and both will produce good negatives -- despite the fact that normal development for the (conventional grain) ISO 400 is something close to twice that for normal development of the ISO 100. I'd usually expect processing "old school" (not tabular grain) ISO 100 film at the same brand's ISO 400 time to give me a two stop push (or N+2, near enough, in Zone notation).

I know monobaths produce a race between fixing and development; this is why most monobaths have shown some loss of true speed, because the shadows develop slowest and fixing tends to over take development for the least exposed halide grains. This is why monobaths have long been considered "niche" developers.

So how do these modern monobaths seem to ignore the "true" process times for different films and produce normal or very nearly normal negatives from films that would normally differ by 2x in process time for that result?

Answer 1

What's commonly left out here is that faster films, generally, have larger halide grains. This is why, for instance, Tri-X has always had a longer development time (same developer and temperature) than Plus-X.

And this same factor means faster films also take longer to fix -- a larger halide grain takes longer to dissolve, from the surface inward. This means that larger grains will have more time to develop in a monobath before the fixing action removes the halide.

Monobaths (especially modern ones based on a rapid fixer formula) use very fast-acting developers: fast enough that there would be risk of uneven development in a tank process (the usual rule of thumb is to make three minutes your minimum). This is offset by the rapid fixing, however, so uneven development isn't a problem with monobaths -- the fixing starts at the same time as development, and if this time varies as the tank fills, it doesn't matter because each square millimeter of film will start both processes together, and fixing regulates development.

In the end, what makes modern monobaths work equally well with, say, Ilford Pan F and Kodak T-Max P3200 is that the longer it takes to develop, the longer it'll take to fix.