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  1. #1
    polyglot's Avatar
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    Colour control with C-41/RA-4

    Hi all,

    I've read up on RA-4 optical printing recently; haven't had the chance to try it yet for lack of chemicals but I hope to change that in the next couple of months and get my colour process all at-home and analogue. However, I have some questions about achieving correct colour balance and how is that even possible with a single adjustment of the enlarger light-colour?

    Compare it to making a scan, where I pick RGB values for both the black- and white-points, thereby changing the colour balance independently for the low and high values in the image. That means I can correct for any mask colour as well as any lighting conditions that the image might have been exposed with.

    Now consider what happens with RA-4; say the (daylight) film was exposed in daylight. You correctly set the enlarger colour and that will result in neutral shadows and highlights, the mask correctly cancels out and everyone is happy. Now what happens when I expose the film under tungsten light? I can dial in a bunch more Y and some M to neutralise the image, but won't it mean I can neutralise either the shadows or the highlights but not both?

    Perhaps I am overthinking this or I'm confused about how the colour mask works; is it actually possible to correct for arbitrary lighting conditions (assuming continuous spectrum, nothing truly nasty) while printing?

    Or must I do all the corrections pre-exposure, just as one would with chromes? Because that would really suck; I can do the colour corrections in a hybrid process but would like the resolution obtainable from optical printing, not to mention a desire to control/implement everything possible myself and that there are images I'd prefer not to send to a lab.

    thanks...

  2. #2
    markbarendt's Avatar
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    Enlarger color corrections are generally global but with some imagination you can say burn the shadows with a different filter set.

    When shooting you can also use colored strobe light or reflected light. This is used regularly by many people regardless of media because it is easier than fixing it in PS or the enlarger.

    With that said it is also important to note that shadows are normally blue, that people under tungsten light really do look more yellow, blah, blah, blah...

    Color correction is a purely artistic manipulation/decision.
    Mark Barendt, Ignacio, CO

    "We do not see things the way they are. We see things the way we are." Anaïs Nin

  3. #3
    RPC
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    Not exactly sure what your question is concerning the color mask. The color mask corrects for dye impurities, and not lighting conditions.

    RPC

  4. #4
    Photo Engineer's Avatar
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    You should not expose a daylight film under tungsten illumination. The proper exposure is obtained only by using a correction filter. This is fully explained in Ctein's book, Post Exposure. I highly recommend it to you as it has photo and sensitometric examples of what you are interested in.

    PE

  5. #5
    RPC
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    In a nutshell, exposing the film to the wrong color temperature causes distortions in the characteristic curves of the film, which cannot be completely corrected during optical printing. Using the proper filter on the camera will prevent the distortions.

    RPC

  6. #6
    Rudeofus's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    This is fully explained in Ctein's book, Post Exposure. I highly recommend it to you as it has photo and sensitometric examples of what you are interested in.
    AND it is available as free and legal PDF download!
    Trying to be the best of whatever I am, even if what I am is no good.

  7. #7
    brucemuir's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    You should not expose a daylight film under tungsten illumination. The proper exposure is obtained only by using a correction filter. This is fully explained in Ctein's book, Post Exposure. I highly recommend it to you as it has photo and sensitometric examples of what you are interested in.

    PE
    In a perfect world this of course should always be standard practice.

    What about mixed lighting where you have no choice?

    Can anyone rif on 4th layer technologies (fuji) and whatever Kodak does to help in these situations?

    I've noticed with tungsten illumination shot on daylight balanced negative stock that newer films handle this MUCH better than in the past? ? ?

  8. #8
    Photo Engineer's Avatar
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    As long as the predominant illuminant is daylight, mixed lighting will work. After all, daylight neg film is similar to daylight pos film if shot under tungsten illuminant. Tests will pretty much confirm this.

    The 4th layer, or any similar technology does NOT help change or modify the balance of film. This technology is directed to fixing the problems with fluorescent lighting which is a different matter.

    PE

  9. #9
    polyglot's Avatar
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    Thanks all; it sounds like it's basically as I expected and no way to completely and accurately fix colour issues post-exposure. Not the end of the earth. Tungsten light is such an extreme example (~3 stops less blue light, the blue-sensitive layer will be completely blank/unexposed and therefore absolutely uncorrectable) so I probably shouldn't have led off with that. What is probably more useful to me is the subtle daylight/shade/flash colour-temp corrections. While I know I can get an 81B/C, it'd be nice not to have to bother.

    (I got a PM saying I should use a colour-print viewing kit. I actually have the Kodak version of that and plan on using it when I start printing, I was just wondering if the process could correct for illumination rather than just film/paper/enlarger combination).

    I'll have to go have a read of Ctein now as I'm lacking a good colour reference.

  10. #10
    hrst's Avatar
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    Basically, you can not adjust contrast as you do digitally with the levels tool. Ironically, you need the levels tool mostly because the digital systems suck and their designers have failed so that you need to fix the problems they have created.

    When the contrast of R, G or B mismatch, it is called a "color crossover".

    Actually, the negative films have very long straight lines with little crossover over a large range. Hence, just by adjusting R, G or B exposure, you can adjust the color balance quite a bit.

    When you have to expose daylight-balanced film in tungsten light (which is not recommended, of course), the key point is to OVEREXPOSE. This is how you can avoid the blue going on the toe. OTOH, there is usually enough latitude before the red shoulders. If you overexpose 1,5 to 2 stops or so, you can correct the balance quite well in RA-4 printing.

    "Tungsten" lights also vary quite a bit, from 2600 K to 3400 K. Photo lights and high-power halogens vary from 3000 to 3500 K whereas lowest-power general purpose bulbs can go as low as 2500 - 2600 K. Daylight film shot in 2600 K in practically uncorrectable, digitally or optically; whereas 3400 K is corrected easily either way.
    Last edited by hrst; 06-12-2011 at 07:21 PM. Click to view previous post history.

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