Quote Originally Posted by mrcallow
In colour neg film red has the least contrast of all channels.
I'm not sure what you mean here, and therefore can't go on to comment on the rest of the post very well, but basically all 3 layers (channels) in a color film have the same contrast.

In color neg, this is about 0.6 for a professional film.

If the contrast of any layer is not the same as the others, then crossover results.

Filters therefore should have equivalent contrast effects, but visually they do not due to the sensitivity of the human eye (perception) and the amount of a given color content in an average scene. For example, there is little red in a noon time sky. Here, I would agree that red contrast is low.

The contrast of a layer can also appear to vary with illuminant, and therefore the variation from tungsten to daylight may appear to be quite different. Even so, the RGB contrasts of daylight and tungsten negative films are all pegged at about 0.6 again. It is a matter of perception by the human eye.

The same effects can be projected into B&W imaging bringing about percieved differences in color object contrast. This is, of course, not true with orthochromatic or monochromatic B&W materials, nor is it true with IR and UV photography. We cannot truly form an objective opinion of light we cannot see. We have to just accept what the film shows us. And the film cannot form a 'real' image of light it cannot see, and so we have to accept the limitations of the film.