Roger and Marty -- in practice, do we see substantial compensation effects with XTOL and films like ACROS that incorporate a lot of accelerants?
Sort-of. Well, as much as with any other comparable film, but to some extent what matters is how long you develop it for and how you agitate. it depends what you call "substantial".
Ascorbates are have development by-products that are acidic. These act as development inhibitors. But you need to give them a chance. In high concentration and/or high agitation environments you swamp any local effects with concentration and/or move fresh developer into that area of the film. My standard process with most of my standard films (neither Acros nor TMX are one of my standard films - I mostly use Plus-X, Tri-X and Neopan 400) and Xtol is to use relatively infrequent, gentle agitation, and high or very high dilution. I use 1+3 as standard and if i need low contrast I go to 1+4 or 1+5 with appropriate volumes and times. With these I can show sensitometrically that there is compensation occurring by changes in the curve shape.
The problem with Acros and TMX is that they need more concentrated developer to work, because the emulsions seem, for want of a better term, like they "need" a lot of developer to be effectively developed to normal contrast. So you get _some_ compensation, and again I can show this sensitometrically, but it is not as much as for other films that you can develop more easily at higher dilution.
The compensation in either film type may not make as much difference in prints or scans, which I have not tried to quantify. I mostly use Xtol because it is sharp, fine grained, economical, reliable and available. I could just as easily use D76, which has a much less lofty reputation but works perfectly well.
How much this has to do with the incorporated development accelerants I do not know, since we cannot obtain Acros without the accelerants to test (and film doesn't work like that anyway).
If you want a real compensating effect, develop Acros in Xtol 1+1 for 70-80% of your normal developing time, then tip it out, replace with Xtol 1+4, agitate for 30s and let it stand for 10-20 minutes. This works, but is a pain, and it is harder to tell if you get "compensation" or just increased film base + fog density because the curve shape becomes really oddly flattened.
I am increasingly trying to ignore *why* things happen and just going with what works, but it isn't in my nature.
Marty