> when we start experience the failure of reciprocity, the shadow part of the curve may become convex or it may become concave, -- it depends how fast drops the film sensitivity. -- Am I wrong?(posted 8170 days ago)If the film is ideal straight line, it is concave and convex. However, if the film loses sensitivity in low exposure region, the actual curve lies below the straight line, making that neighborhood concave. When you look at reciprocity failure at the other extreme, at high exposure, the curve again lies below the ideal straight line, again making it concave. What I think you are confused is that contrast is related to first derivative while concavity is related to second derivative. But let us not talk much about mathematical analysis here.
> may be it makes sense to split the development into 2 baths: 1st does mostly sa elective hardening, the 2nd does mostly the very development?
It makes sense, but we have to make sure that the second developer does not soften the gelatin again.
Even with modern films, highlight areas require increased intake of chemicals and build up more reaction products. It's probably that the absolute amount of chemical needed has decreased compared to the era of Super XX, or gelatin's diffusion became so efficient, or gelatin can hold enough chemicals to survive between somewhat infrequent agitation. A simpler approach, perhaps with lesser effect, is to further slow down the diffusion of chemical across the gelatin. I am more comfortable to let statistics of kinetics control this instead of letting a magic hand controlling fluid turbulence. One rough indication for whether the rate of diffusion is mainly limiting the rate of development is as follows: Take a developer X (1+0), X(1+1), X(1+3). Let t(n) be the time required to maintain certain contrast criterion at 1+n dilution. If t(3) \approx 2 t(1) \approx 4 t(0), development is nearly perfectly diffusion rate limited. When the concentration-time curve is plotted, the closeer to linear with slope 1, the larger the contribution of diffusion rate to the overall rate. (This does not take initial gelatin wetting time into account, so be aware of that.) What does it tell you? Microphen combined with HP5+ is largely diffusion limited. This (HP5+ in Microphen 1+4 or something) might be worthwhile testing with somewhat infrequent agitation Ted suggested, possibly with pre-bleach as Andrey suggested, which I think is a good idea. Microphen should be pretty resistant to bromide drag (because of phenidone at low pH and added bromide). Incidentally, I have been trying a similar idea with different approach for different purposes. I'll report it at another thread, because I haven't considered compensation in that formula.
If there are other suggestions, please post them here. When the thread dies out, I'll summarize, edit and give it to Ed B.