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? about using curves for color correction

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  #1  
Old 11-23-2003, 06:59 PM
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rxdan rxdan is offline
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? about using curves for color correction

I am new to advanced photo editing and have a question concerning using curves for color correction. Could anyone explain how to determine reference values for the color correction curves for the bleedingheart.psd picture (in the HPPE2 page 103)if the grayscale reference card was not there? I can find a spot in the photo using the eyedropper that registers pure black (0,0,0) but I do not understand how that helps me determine one or more reference points to correct the color of the individual channels. I get the picture to turn out fine by adjusting the points I know should read 25,50 and 75% gray, but if I did not have those points, I would not know where to begin other than just trial and error.

Thanks,

Dan
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Old 11-24-2003, 06:07 AM
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Richard_Lynch Richard_Lynch is offline
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You need to photograph the image with the gray reference in it (hold it in the frame while taking the picture). The card is assumed to have RGB values of 64,64,64; 128,128,128; and 191,191,191, representing 75%, 50%, and 25% black. You determine the values as it discusses in the text, by using the eyedropper and Info palette to sample the grays and take down the numbers. Once you have made the samples, you have to go through the calculations to make the adjustment, using Curves.

If you have run through the entire section from 99-105, can you point to the spot where you are unsure what is happening?

Richard
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Old 11-24-2003, 10:07 AM
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Richard,

Thanks for your reply. I understand the process (amazingly enough) but I was asking in terms for getting a basic understanding for using curves for color correction in photos where you do not stick a grayscale card in the frame of your picture (my vacation photos). I think maybe I will just post the picture in the user help this picture section and see what kind of feedback I get.

Thanks again,

Dan
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Old 11-24-2003, 11:21 AM
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Richard_Lynch Richard_Lynch is offline
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The book also discusses correcting for assumed grays. There is no real way to make a curve correction without a reference -- otherwise you are essentially guessing or using trial and error.

Feel free to post the image here.

Richard
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