RGB filtering for CMYK separated negatives for gum printing?

[Jadedoto]

Hey everyone!

So this weekend I've been doing a workshop with Christina Z Anderson learning how to gum print (in addition to my alt processes class), and I've completely fallen in love with gum. We're doing digital negatives, but being the die-hard film kid born several decades too late, I wanted to try out making CMY negatives using my 4x5.

I've tried scouring about on APUG, LFF and on the Great Google, and have basically found two responses:

1: Shoot color, print separate negatives in the darkroom
2: WTF why aren't you using digital.

The first answer is that I just want to use BW, the second answer is obvious.

I'm going to see how well this works, today I shot some exposures and will be printing them tomorrow, and this is what I shot:

1 negative through 60CC Red
1 negative through 60CC Blue
1 negative through 40CC Green
1 negative 3 stops over (N+3) for Key (Black)

I also have a straight, unfiltered shot. All these were shot on FP4+ and in Sprint Standard Dev (I was out of my normal D76, so I used what we had at school :\ ).

The filters I used, because it was spur-of-the-moment, were some color correction filters from the color darkroom, and I just held them in front of my lens while shooting (trying to avoid glare as much as possible). I looked at the negatives tonight and they do look slightly different, so I think I'll at least get some pastel-y coloration on the gum print.

I'll post more results plus the whole process of the print when I get more done. Has anyone else done this recently and have any advise on how to make it work? (I remember seeing one post on APUG where PE just suggested the depth to the filters corresponds to the vivid-ness of colors, which makes sense).

Thanks!
-Vincent

 

[Mainecoonmaniac]

You're doing in camera separations. I think you need sharp cutting filters same as dye transfer. Hope this helps.
http://www.charlescramer.com/dyetransfer.html

I remember seeing an old camera used before the days of color film that did separations in camera with one shot. If I remember correctly, I think it took 3 sheets of film. One for each color.

 

[gmikol]

More specifically...I think you're looking for wratten #'s, 25/58/47, or 29/61/47B, or 70/99/98 (R/G/B for each set), or modern dichroic (glass) R/G/B filters, which have very sharp cuts. Keep in mind that your exposures for each color will be different.

Good luck...

--Greg

 

[2F/2F]

As well as separating in camera, you can also separate from a positive original. This will make your life much easier when shooting, as you will not need to shoot four shots per composition (eight shots per if you are shooting backups), and it also will eliminate any of the color ghosting that occurs if there is any movement between exposures. Shoot chrome film, and use the proper color separation filters on your enlarger. CC filters definitely will not cut it. Wratten numbers 25 (red), 58 (green), and 47 (blue) make up one color separation set for working from positive originals. There are several other sets as well, though I do not know them off hand, or how they differ.

 

[hrst]

Also, if you have dichroic color enlarger head, you can use it to make the separations from chromes. Maximum M&Y, maximum Y&C and maximum C&M. At least the enlarger I use produces pure enough R,G and B light easily. (Well, what is pure enough? It's not necessarily as good as those special sharp cut wrattens, but comes very close in most practical purposes I'd guess.) You have to experiment with R,G,B exposure times to adjust the color balance.

40...60CC doesn't sound pure enough, but of course anything that looks like red, green and blue work at some level, just giving subdued saturation due to color channel crosstalk.

EDIT: Oh, this was the Usual Answer number 1. Sorry, didn't read the OP too well. If you are going to shoot tri-color in camera, then you should not use sharpest-cut filters. You can use sharp-cut filters to get a more saturated, unrealistic look, but some colors will be too dark. Normally the in-camera spectral responses of R,G&B should overlap so that single-spike yellow and cyan do not end up black, but expose both of the records (R&G, or G&B).

 

[2F/2F]

It all depends on the accuracy you are after. The dichroic head would produce a softer separation than the true color separation filters. This could look very beautifully inaccurate in a gum print.