Over the past year, I've been trying various combinations of D-23 and Thorton's divided developers. Most seem to cut the film speed in half - shadows go blank - when compared with Xtol 1:3. Is this normal? Should I increase the time, agitation, or amount of metol? Is there a divided/two bath developer that gives full film speed?
James
Two things are happening.
First, two bath developers are probably not giving enough development to fully develop the shadows. While there is the old truism that we develop for the highlights, that is only true to a point. Without sufficient development, the low densities never build as far as they should.
Secondly, XTOL (and especially, 1+3) is the most efficient commercial developer for shadows. Even you went a little further, and extend your development time a bit, and reduced your agitation to 10 or 15 seconds every 3rd or 5th minute, you would literally PUSHING the shadows while PULLING the highlights, resulting in very full shadows, gentle highlights, and midtones that were exactly right.
D-23 (and D-76) are efficient shadow developers, yet with many films (Tri-X, TMY2, etc) Xtol is 1/2 to a full stop faster.
In short, you can improve your results with various 2-baths, but never approach what Xtol gives you naturally.
One of the reason folks go on about
minimal agitation, not standing development, is that it lets you fully control, and balance, the shadows, midtones, and your highlights.
Two bath development was investigated in the 1930s to see if an shadows and highlights could be balanced, yet the researchers found little evidence of success. The technique proved useful for many types of work, and with some developers, yet the forgotten assumption to the work was treating agitation as a constant.
Allowing agitation to be a variable gives a careful worker that necessary degree of freedom to get the very most out of a negative, and for roll film shooters, to maximize the quality of the negatives or a roll shot under very different lighting conditions.
And THAT is what the two bath method has tried to do. Trust your evidence. You may lengthen the time in solution A, and get better results. Or, you can take the results from Xtol, and fine tune them.
g-luck