Yes the explanation I gave was somewhat simplified for practical purposes. But in the end it is always about modulation of blue and green light hitting the paper. That is what the filters are doing, whether you use a single filter, or combine hard and soft filters in some exposure proportion to "build" the filtration of a single intermediate filter. As I said though this equivalence of course only applies in the absence of local manipulations during hard and soft exposures.

The principles are the same for additive colour mixing with blue and green filters (Beseler Universal VC heads for example). It's just a different way of getting blue and green light to the paper.

Quote Originally Posted by Rafal Lukawiecki View Post
Michael, while I follow your statement, I wonder if you would agree that there is a more detailed explanation of how VC papers work? I would humbly observe that it seems, based on their characteristic curves, that the component emulsions are differently sensitised to the blue and green light and that it is their additive result that creates the actual, observed contrast. Further, modern VC papers, like Ilford (or like Polymax used to) seem to have three emulsions, with the third one being sensitive to both green and blue.

Unfortunately, this can also lead to some odd behaviours at extreme low grades, such as 00. See Nicholas Lindan short paper: "The Workings of Variable Contrast Papers and Local Gamma". For that reason, split-grade technique can be a little easier to use with filters a little harder, such as 1 and 5, rather than 00 and 5. I believe that is what Bob Carnie practices. The effect will be the same, but the observed changes will seem more logical when using 1 rather than 00 for certain mid-tones.