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Thread: Blown Red Channel...

  1. #1
    jprzybyla's Avatar
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    Blown Red Channel...

    Recently I was photographing Poinsettias outside next to our house. Checking the histogram of the color channels I saw that the red channel was climbing the wall on the right side. Later checking other photographs that had the color red in them I noticed the same thing. Once that happens detail is lost in the red, the same as the brightness channel recording detail as a white blob. Any suggestions from experience how to handle the red channel blowout?

  2. #2
    dje's Avatar
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    Re: Blown Red Channel...

    Joe, Following on from the discussion in this thread, I would say for existing files, if they are jpeg there is nothing you can do and if they are RAW you may be able to recover the red clipping with the Exposure or Recovery sliders in ACR. (Not sure about other RAW software).

    If you want to avoid the problem on new shots, reduce your exposure.

    Cheers Dave

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    Moderator Dave Humphries's Avatar
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    Re: Blown Red Channel...

    As Dave says "reduce your exposure"

    Most cameras have an RGB histogram and it only takes seconds to review last shot, spot the problem, dial in a bit of -EC (if semi-auto) and reshoot - that's what I do.

    Cheers,

  4. #4
    Glenn NK's Avatar
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    Re: Blown Red Channel...

    I've had this problem a number of times with red flowers. The RGB histogram is essential for these - if the histogram is "climbing the wall" then it's probably too late and will mean a re-shoot.

    Glenn

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    Re: Blown Red Channel...

    Happens too with yellow. So when I have either or both these colours in the composition and depending on their dominance I will reduce exposure to suit the lighting conditions but many times I forget.

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    Glenn NK's Avatar
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    Re: Blown Red Channel...

    Quote Originally Posted by Bobobird View Post
    Happens too with yellow. So when I have either or both these colours in the composition and depending on their dominance I will reduce exposure to suit the lighting conditions but many times I forget.
    I've had far less trouble with yellow perhaps because it is comprised of two colour channels rather than one (like red).

    If a colour is comprised of two of the RGB channels, I would think it would be less likely to blow out as the two channels share the light as it were, making clipping less likely.

    Glenn

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    Re: Blown Red Channel...

    As far as I know, the light meter is less sensitive to red (or not taking it
    into account as much when calculating the exposure), so an intense
    red isn't seen as very intense by the camera compared to the green
    next to it (for which the metering is much more sensitive). Result:
    red channel gets over exposed. For an intense yellow, both red and
    green are present at high intensity, so the camera (exposing for the green)
    won't over expose the red.

    Then there's the white balance correction, which works by scaling the red and
    blue colour channels relative to green, which can also cause clipping of one of
    the channels (at least dcraw, used by a lot of raw converters under the hood,
    by default uses scaling factors larger than 1)...

    Remco

    edit: reformatting (problem seems to have solved itself)
    Last edited by revi; 4th December 2011 at 03:35 PM.

  8. #8

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    Re: Blown Red Channel...

    Thanks for the explanation. Just to be on the safe side will stick to somewhat underexpose if yellows dominate the scene or are throwing back strong reflections.

  9. #9
    Moderator Dave Humphries's Avatar
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    Re: Blown Red Channel...

    Quote Originally Posted by revi View Post
    edit: reformatting (problem seems to have solved itself)
    Except there are still hard line breaks in it (instead of the text going the full width)

    I know we are having a few editor related problems, could I ask which browser you were using?

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