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  1. #1
    pellicle's Avatar
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    filters - colour absorption question

    Hi

    pardon me, but I'm not sure where's the best place to ask this, so I thought I'd start here.

    I normally print black and white, and I have multigrade filters for that, folks who have experience in colour printing will no doubt understand the filters better than me, so here I am.

    If I was wanting to absorb more of the green and some of the red spectra from a light source what colour filter would I use? I know that a blue appearing filter will remove more of the red and green, but what colour will a filter which removes some of the green as well appear?

    Thanks
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    For printing colour? The colour of the filter removes the same colour. So more green in the filter means more green removed.

    Or do you mean something else?

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    pellicle's Avatar
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    Quote Originally Posted by Nick Zentena View Post
    For printing colour? The colour of the filter removes the same colour. So more green in the filter means more green removed.

    Or do you mean something else?
    nope, I didn't mean something else. That's what I meant .. so a filter which removes some blue, and some of the green would look what colour?
    Theory: you understand why it should work but it doesn't
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  4. #4
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    I don't think it's very useful to think of 'removing' colour. It's easier to think in terms of what is transmitted. A filter has a certain colour because that colour is what is transmitted. A red filter transmits red... and blocks the other colours.

    So... a filter will simply change the relative weights of the colours. If you use a filter (say, red) then the light transmitted through that filter will be more red weighted: there will be less green, less blue... relative to red. If you want more blue and green transmitted then you could use an orange or yellow filter.

    You could also do a multiple exposure: do one shorter exposure with all the colours (no filter) and then another exposure with a filter for the one you want to dominate.... that would allow you to tune the relative exposures very finely with time.

    P.S. my post is at odds with Nick's assertion that a green filter will block green; not sure what Nick means, let's see...
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    If your colour print is too green adding green to your filter will remove the green. With normal RA-4 printing. I guess this is subtractive?

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    Oh you are talking about RA4.
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  7. #7

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    For RA4 printing:
    Too green - subtract Magenta
    Too Cyan - subtract Magenta and Yellow in equal amounts
    Too Blue - subtract Yellow
    Too Yellow - add Yellow
    Too Red - Add Magenta and Yellow in equal amounts
    Too Magenta - add Magenta

  8. #8

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    If I was wanting to absorb more of the green and some of the red spectra from a light source what colour filter would I use? I know that a blue appearing filter will remove more of the red and green, but what colour will a filter which removes some of the green as well appear?
    Pellicle, from the simple viewpoint that we often take as photographers, we consider white light to be a mix of some reddish, some greenish, and some bluish light. From that viewpoint, and considering your desire to remove (filter out) both some green and some red light, you would want to let the third color, blue, to come through unaffected. since mainly blue light is coming through, if you look through that filter, toward a white light), the filter appears bluish. But it sounds as if you already know this part.

    Now, if you want to remove, for example, two units of green and only one unit of red (this is what you are asking, I think), here is one way to do it: first, use a blue filter with enough strength to remove both one unit of green and one unit of red. Now it remains only to remove one more unit of green. It turns out that such a filter, a green absorber, exists under the name of magenta. So your final filter pack would consist of the combination of blue filter and magenta filter. And if you hold it up to the light, and look through it, it has the appearance of a magenta-ish blue, or a bluish magenta, whatever you wish to call it.

    Now, if you didn't know what magenta was, I'd describe it as a filter which blocks green, allowing red and blue to come through unscathed. So you could now see that my previous filter pack, blue plus magenta, is roughly equivalent to a double blue filter plus red filter, having a slightly reddish-blue appearance. Making any sense?

    If you try to understand filters any deeper than these simple ideas, I think you need to look at spectral data. But for color printing, and using filters with only partial absorption, the ideas of red, green, and blue, combined with their complements of cyan, magenta, and yellow, work perfectly.

    With regard to color printing, a rule I learned long, long time ago, which seems virtually infallible, is simply this, "Always do the wrong thing." If your print is too green, it seems like the wrong thing is to add green filtration. But do it anyway, it will work. Except, we don't normally use green filter, so you do the equivalent thing, which is to REMOVE magenta filtration. (I said it was equivalent, but actually it changes the exposure a bit).

    A bit more comlicated example, per Bob-...'s "Too Cyan..." example: If your print is too cyan, you want to do the wrong thing, which is to add more cyan. However, conventionally, we don't adjust cyan; you want to do the equivalent thing, which is to remove red. However, we don't usually handle red filters, either, so you jump to the equivalent of removing both magenta and yellow. Once you work with this a bit, these equivalents will become second nature. In the meantime, you might want to consult that diagram, with three overlapping circles, labeled red, green, and blue. The little section opposite red is cyan, opposite green is magenta, and opposite blue is yellow.

    Did I make this too long and complicated? If so, just remember, "always do the wrong thing."
    Last edited by Mr Bill; 09-02-2009 at 06:27 PM. Click to view previous post history. Reason: spelling

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    Wow, kinda makes black and white sound pretty attractive.

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    Wow, kinda makes black and white sound pretty attractive.
    hehe...but think about it Tim, with B&W, you have to do everything right. But for color, just use my method, to always do the wrong thing. If you know zilch about color, almost for sure you'll do things wrong, right? So in no time at all, you'll be making great color prints!

    In truth, though, it's not as difficult as it seems. Once you work through that color diagram a few times, things fall into place.

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