Making Color Separations

Discussion in 'Color: Film, Paper, and Chemistry' started by jantman, Apr 29, 2004.

  1. jantman

    jantman Member

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    I've been thinking about trying an alternative process. Though I know that I'll begin to dabble in Cyanotype soon, I'm particularly intrigued by the various color processes. Most of them involve obtaining color separations of the original neg. Could someone please explain to me exactly how this is done (more specifically, what filters and films)?

    I know, obviously, I would have to use some sort of panchromatic copy film, I'd aim for whichever one Arista makes (I forget the film name). What is the exact process, either enlarging or by contact?

    Thanks!

    (Given what the cost would probably be, I should stick with my dreams of Dry Plate).
     
  2. Greg Davis

    Greg Davis Subscriber

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    I have been looking at dye transfers now that the Efke brand materials are available in the US. The process for color separations is described in the Kodak publication for the dye transfer process. This publication was available as a PDF download, but I couldn't find it to give you the URL.

    What you do is take a color slide and expose it to three separate sheets of black and white film using an enlarger. You can use any white light source, but an anlarger lets you control the light better. Each sheet needs to be exposed to a different filter. One is exposed using a #29 red filter, another with a #58 green filter, and the third with a #47b filter. These are then developed as normal black and white negatives. You need a densitometer to see if your exposure and development was consistent with each color in the neutral shades like black shadows and white highlights. You are supposed to plot the gamma of each to see if it matches the others. If you can find this download, it explains it in excrutiating detail.

    Bear in mind that a color negative is a color separation. But another way to make these is to photograph the same scene three times but use the three filters for each respective shot. That way you bypass the color film altogether.
     
  3. jantman

    jantman Member

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    I think I may have seen that PDF at one time, I'll see if I still have it. I think maybe I saw it mirrored somewhere.

    Well, I guess I'll have to pick up a densitometer sometime...

    The problem with shooting separations is it prevents me from still doing normal (Ilfochrome) prints.

    If this is done as a contact method, I assume that the filter would be just placed over the lightsource? I'm mainly interested in working from 8x10 transparencies.
     
  4. inthedark

    inthedark Member

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    Well, you mostly seem to have the information here, but I would make some suggestions (this is the pre-press type stuff my copy cameras were made for.

    1. Use orthochromatic film when using the blue and green filters and save the panchromatic for the red layer.

    2. The negative densities do NOT need to match the slide or print for each color because when you print them back with color paper, you control the each color by exposure. Usually the three negatives are almost identical in density and the print back exposure tends to run about 15-20 seconds for the green and blue print back, and about 45s for the red, or a similar ratio.

    3. If you know how to use half-tones screens, all of the negatives can be accomplished by lith film.

    4. Most computer image editors now a days will provide you with a very reasonable halftone print in separations and these can then be "inverted," printed on clear film and used quite nicely. This may take some trial and error, as this process as with most processes will tend to gain on contrast if not pulled back just a bit.