Quote Originally Posted by chuck94022 View Post
So, my question is this, specifically for a divided development based process: why would there be crossover, if the developer works to completion in all layers? Wouldn't all the color layers be fully developed, and thus balanced?

I think the simplest response is to say that no, the developer does NOT work to completion. If you doubt this, consider the situation of people who "push process" C-41 films. They use longer development times to increase the amount of development, which proves that the spec development is NOT to completion.

The development of C-41 films is an incredible balancing act between the processing chemicals and interactions within the emulsion, optimized for the spec conditions. So it should not be surprising that any variation might produce color crosses. I'm not sure how someone would measure these sensitometrically, except in the simple case of a neutral tone scale (photograph a gray scale, or the equivalent via a sensitometer). Even if no color crosses exist here, who can say if such would exist in a non-neutral case, such as human skin tones in portraiture?

I've done pretty extensive testing on pro color neg films for studio portraiture, doing evaluations for a large chain/finishing outfit. When new pro films were being introduced, we would start with sensitometric screening and simple shooting tests. These were followed up by exposure variation tests, where all would be optically printed onto the matching professional papers; the prints are hand-balanced to match one master control print, roughly to skin highlights. Professional color correctors, using special color booths, would evaluate the results, both critically and with respect to what an average person might notice. The results are too complicated to put into a written report, other than superficially, and certainly a densitometer is not adequate for this. If future questions should arise, we'd always pull the original test prints back out; maybe the test has to be repeated because of some other changes.

Those days are long gone, and modern digital printing can probably, in principle, be adjusted to correct nearly any deficiencies.