Less green

Good morning from french
I would like to ask you if it is possible to desensitize the the green color response on a film, eg. treating it with a desensitizing bath, a Fuji Pro 400H -135 film or its Kodak equivalent, without interfering too much with the sensitivity to the other colors;
In practice I would like to get green or very dark or very gray.
Merci beaucop...

What specifically are you after? I suspect that if you want less green response, the easiest way to achieve this is to put a magenta filter on the front of your lens.

(Also, I'm suggesting this thread by moved to the Color: Film, Paper, and Chemistry forum, since it's not really about making emulsions.)

Thread moved.

I agree, it would be much simpler to filter the film to whatever sensitivity you want, either with a filter on the lens or at the printing stage, than to try to desensitize one layer without changing anything else about the film. How could you change the sensitization of one layer, for instance, without removing the antihalation layer or antistatic treatment, not to mention changing the sensitization of the other color layers?

Yes, filter the incoming light (with a lens or gelatin filter), or make the necessary changes in the editing steps involving electricity and software. The D-word.

Easy to do in darkroom printing by registering a pan film mask exposed using 58 or 61 filter green light.

... a bit more complicated if printing a color neg, cause you have to add something like a 15 yellow-orange filter too, to null out the orange mask. But I won't elaborate unless someone is seriously interested in this approach.

DREW WILEY said:

... a bit more complicated if printing a color neg, cause you have to add something like a 15 yellow-orange filter too, to null out the orange mask. But I won't elaborate unless someone is seriously interested in this approach.

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Would be interested to hear more. I take it you can do the same thing with the other separation filters if you want to affect the colour reproduction elsewhere? Which film are you using as the masking film?

FP4 works well for masking sheet film originals, but TMax has finer grain and works better for small originals. Use a 5-mil sheet of frosted mylar between the two films, emulsion toward emulsion in a contact frame. Dev in highly dilute HC-110, 1:31 from stock, which is itself 1:3 from concentrate. The dev time and amt of contrast & density depend on the film type and just how much you want green deliberately over-masked. If the exposure climbs too far, you can edit the thinner areas in a minute or two in Farmers Reducer. Not at all difficult in principle, but takes some experimenting to judge the degree you need. You can also increase dev conc to get more density. If you do a lot of this, a punch and registration frame are highly recommended. Otherwise, visually register on a lightbox, and tape the two films together for printing.

DREW WILEY said:

FP4 works well for masking sheet film originals, but TMax has finer grain and works better for small originals. Use a 5-mil sheet of frosted mylar between the two films, emulsion toward emulsion in a contact frame. Dev in highly dilute HC-110, 1:31 from stock, which is itself 1:3 from concentrate. The dev time and amt of contrast & density depend on the film type and just how much you want green deliberately over-masked. If the exposure climbs too far, you can edit the thinner areas in a minute or two in Farmers Reducer. Not at all difficult in principle, but takes some experimenting to judge the degree you need. You can also increase dev conc to get more density. If you do a lot of this, a punch and registration frame are highly recommended. Otherwise, visually register on a lightbox, and tape the two films together for printing.

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Cheers for that, the only thing I don't have on hand from that list is the HC-110 - I take it you can do similar things with the red & blue separation filters too?

Just take a color chart or colorful print and look at it through different color filters to see what is darkened or relatively bright. Panchromatic film is analogous, but certainly not identical to human vision. If you can't get HC-110, try D76 or ID-11. Not as good for low-contrast masks, esp w color neg film, but might work OK for chromes in your present application. Try 1:1, 6 min, 20C.

Drew, can you please explain, how a B&W mask can possible suppress one color channel? Your masking technique may work, but it would require (ideally orange mask less) color negative film, not FP-4 or TMAX.

He's trying both both darken and selectively mute or gray the green hues in the final image. In a transparency, green is green, so you make a contact neg or mask of it thru a deep green filter on b&w pan film, overdo it a bit, and the max film density will correspond to green. How you print it is a secondary question, but I presume a mask could even be taped on for a flatbed scan. .. With color negs, intense greens appear magenta on the film, but because everything on film prints counterintuitively on RA4 paper, in reverse, negatively, you still expose through a deep green filter, but in this case, it's more complicated than I care to describe here. Best to get to first base first, when learning masking technique.

DREW WILEY said:

He's trying both both darken and selectively mute or gray the green hues in the final image. In a transparency, green is green, so you make a contact neg or mask of it thru a deep green filter on b&w pan film, overdo it a bit, and the max film density will correspond to green. How you print it is a secondary question, but I presume a mask could even be taped on for a flatbed scan. .. With color negs, intense greens appear magenta on the film, but because everything on film prints counterintuitively on RA4 paper, in reverse, negatively, you still expose through a deep green filter, but in this case, it's more complicated than I care to describe here. Best to get to first base first, when learning masking technique.

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My own application would be for optical RA4, really just something to play around with when I'm not stupidly busy... And getting HC110 isn't an issue for me - it's just I need to remember to order some in!

If scanning & then reworking the image digitally, a selective colour layer in Photoshop can be used to do not dissimilar things to what you are describing photochemically - though I'd avoid trying to keep a mask in register during a scan, just get the best scan that can be afforded & do the work in Photoshop.

You can generate a mask in PS and print it out, but registration will be less precise than by contacting direct to film, which is actually a lot less work and more precise once you understand it, and can also be processed in small batches if desired. The larger the film format, the more precise it's likely to be unless you have a punch and matching registered contact frame. But you can learn the basics without that. For your application it might be easier to first make a contact interpositive through a strong magenta filter like a Wratten 33 or 34, then use that to generate a negative contact mask where the greens in the original are the least dense on the neg, and therefore create the most density in the RA4 print. This is fun stuff once you get used to it.

You could also use Ortho Litho film for the final second-neg mask, or even use a staining dev on FP4 to selectively muddy the greens. Or you could do the whole thing reversal processing using just one b&w neg. Lots of fun tricks you could try.

DREW WILEY said:

He's trying both both darken and selectively mute or gray the green hues in the final image. In a transparency, green is green, so you make a contact neg or mask of it thru a deep green filter on b&w pan film, overdo it a bit, and the max film density will correspond to green. How you print it is a secondary question

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The printing/scanning stage is where this technique will break down: you create a mask through a green filter, this mask then has density in all the areas which have no magenta density in the negative, i.e. areas in the original scene with no green light. That mask will now coverall color channels in all areas in which had no green light in the original scene, i.e. a bright red light will be covered by the mask and will render white in the enlarged image. Unless my reasoning here is out of whack, I am fairly confident that this is not what the OP wanted.

Here's the problem with masking: OP wants green light to be invisible. Green light will, however create magenta density on the negative. In order to make the green light invisible, one would have to create a mask with negative density, or one with very high overall density in yellow and cyan and variable density in magenta. Even if you get it right, you are looking at a mask which gives you at least D=1 extra density, I'd hate to think about enlarging these masked negs to bigger formats. If an extra D=1 is not an issue, how about this: expose the negative to strong green light before development, then develop, then during enlargement filter away the extra magenta density. Since H&D curves are logarithmic, this trick should clamp down all but the brightest green areas of the scene. Be prepared for crazy RA-4 filtration values.

About desensitizing one layer: you can specifically reduce density in the top most layer by adding measured amounts of iodide or PMT to your developer. However, at least with Fuji films, the green layer is not the top most layer, so that's not an option here. Killing the sensitizer dye after exposure would be useless: the exposure has already happened, the sensitizer dyes have already done their work and created developable silver specks. You'd have to attack the silver specks - not an easy thing to do with any but the top most layer.

I already gave a valid set of methods for how to do it entirely in the darkroom. But masks can be generated from scans if necessary, then attached for darkroom printing. But film to film contact is more precise. I said nothing about digital printing. And I've done this kind of stuff myself for three decades now.

DREW WILEY said:

You could also use Ortho Litho film for the final second-neg mask, or even use a staining dev on FP4 to selectively muddy the greens. Or you could do the whole thing reversal processing using just one b&w neg. Lots of fun tricks you could try.

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Cheers, will give those a try at some point!

Prior to scanners and PS etc, entire industries - people in pre-press and ad work, and high-end color printing in general - routinely were expected to understand how
to solve problems like this using film. It can be fun reading some of the old manuals for general concept, even though specific films and techniques differ now. One
place you can find a representative collection of old masking literature and even videos is on David Doubley's site under Dye Transfer. Just as soon as I've finished breakfast, I'm headed out to the shop to complete a new phenolic pin-registered 8X10 film easel, just as the rainy season ends and I'm out with the camera more.