rogue_designer said:
This may be a bit simplistic, but I've wondered (and have no way to experiment to see)... if it would be possible to place a Cyan/Blue filter under the enlarger to negate the Orange mask.
Longer exposures certainly, but would that cure the contrast issues caused by the orange mask?
In short: no, because of subtractive colour mixing.
I tried that. The problem is the sensitivity of the paper to different wavelenths. Variable contrast papers tend to consist of two different emulsions, a low-contrast emulsion that is sensitive to green and a high-contrast emulsion that is sensitive to blue. By varying the colour of light, the emulsions get different exposures, resulting in variable contrast overall.
Orange is the complementary colour of blue. Your enlarger head emits white light in a continuous spectrum. That goes through the orange mask of the film, which filters out all the high-frequency bluish light and leaves only light with longer wavelengths. In theory, filtering this orange light with a blue filter should produce no light output at all. In practice, because no filter is perfect, you still get some light output, but a blue filter can't add any blue light to what comes out of the orange filter of the mask. This is because of the way filters work, which is crucial to understand:
a blue filter doesn't work by adding blue-coloured light, but by blocking all the differently-coloured non-blue light. If you filter orange light through a blue filter, it doesn't get blue, it gets dark instead.
If you try to print Kodak BW400 with its orange mask on variable-contrast paper, the blue high-contrast emulsion doesn't get much exposure, because the orange light from the negative doesn't contain much blue light anymore, while the green low-contrast emulsion gets a lot more. As a result, contrast from such prints tends to be very low. The fact that you can get "decent" contrast (i.e. contrast equivalent to an Ilford #2 filter or so) out of a picture at all is only due to the imperfect filtering done by the orange mask, and in order to reach #2 contrast in the picture, you have to use a #4 or #5 filter and live with comparatively long exposure times.
You do get results that way, but I don't find it very satisfactory because you have very limited control over the contrast of the final print. #4 or #5 contrast on the final print is pretty much impossible to get, unless you use a LED colour head with monochromatic blue light (so that the green emulsion does not get any exposure at all) such as blue LEDs, and then you have to live with
very, very long exposure times, hoping that the orange mask will allow some of this blue light to pass.
Fixed-contrast paper is no solution either, because the emulsion has different sensitivity; it tends to be sensitive only to high-energy, low-wavelength bluish light, which is why you can use a greenish darkroom safelight if you use fixed-contrast paper exclusively. Since bluish light gets filtered out by the orange mask, you would have to use very long exposure times. I tried that, too; getting a print on fixed-contrast paper with a high contrast specification is a lot of trouble.
The only alternative is panchromatic paper which is sensitive to orange as well. The problem with that is that it's (a) hard to get - there used to be some by Oriental, but it's discontinued - and (b) there is no contrast control, because the paper is sensitive to all frequencies, and (c) you have to work in the dark, because the paper is sensitive to the red light from your darkroom safelight. IMHO it's just not worth it. So I stopped using BW400. Now I stick with XP2, where you just don't have these problems because the mask isn't orange.
Philipp
