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Thread: Correcting colour casts

  1. #1
    Join Date
    Feb 2011
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    Canberra
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    Default Correcting colour casts

    Sometimes the colour of your image is not what you expected. Shadows & cloud can make things a bit blue. Sunrise and sunset impart an amber glow. Green foliage itself can reflect green light on to your subject.
    For those whose version of Photoshop or other post processing software allows conversion to Lab mode, there is a very neat and accurate way to correct the colour cast.
    This is a tutorial I sent to Rick Playle using one of his images, and with his permission, and that of the administrators, I am posting it here too.

    Colour correction using Lab
    1. Windows Info Panel.
    a. Keep the info panel open and to the side of your work all the time.
    2. At the top right-hand corner of the info panel is a drop down menu icon. Click on that and select Panel Options.
    a. Make sure either the first color (sic) readout or the second color readout is set to Lab.
    3. Move your cursor over a section of the image that you know should be exactly white, black, or neutral grey.
    a. If the “a” channel is -2 or further away from 0, your image has a green cast. If it is +2 or further away from 0 it has red cast.
    b. If the “b” channel is –2 or further away from 0, your image has a blue cast. If it is +2 or further away from 0 it has a yellow cast.
    4. Convert your image to Lab mode. On CS4 this is through Image Mode Lab.
    5. Open curves through Windows Adjustments curves.
    a. Click on the hand icon in the top left side of the panel.
    b. Shift-click on a couple of parts of your image that you believe ought to be actual white/black/neutral grey. This should leave a little marker for you to go back to. Also, the readings for these points appear at the bottom of the Info Panel, and show you the before and after readings as you change the colour.
    6. From the drop-down menu at the top of the curves panel, select the “a” channel, and click on the point where the diagonal line crosses the centre of the graph. This must be exactly in the centre as shown by 50/50 showing in the “input” “output” boxes below.
    7. If the image is grossly over or under exposed correct that first in the lightness channel. (Yours was underexposed and I lightened it)
    8. Then go into either the “a” or “b” channel. Click on the centre point and using the up and down arrows, move that point up or down until your neutral point reading gets close to 0. Do the same in the other channel.
    9. You will notice with your image I still have a strong colour cast in the black area (marker 2), but since black is supposed to be strongly coloured, I am less worried about that. I have managed to get the white marker (1) and grey marker on the grey stick (3) close to 0, and that is about the best one can do.

    Click image for larger version. 

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    Margaret Leggoe
    Never too old to learn.

  2. The following user says thank you to Margaret Leggoe for their reply:

    Rick Playle (15-06-2011)

  3. #2
    Join Date
    May 2010
    Location
    Narrogin Western Australia
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    Default

    Thank you kindly Margaret for your great effort in putting together this infomation.
    Guys although I dont have the same version of photoshop as Magaret this info has given
    me mighty fine insight on what to do and is a great benifit to me and others. I'm sure you will agree
    kind regards Rick
    Through the lens behold the glory of creation
    So much to see, So little time to behold
    Other captures located here :
    http://www.redbubble.com/people/justrick
    https://www.facebook.com/rick.playle.5

  4. The following user says thank you to Rick Playle for their reply:

    Margaret Leggoe (15-06-2011)

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