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Thread: Photoshop RAW Histogram Question

  1. #1

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    Photoshop RAW Histogram Question

    While adjusting an image in Photoshop RAW filter I keep an eye on the histogram. If one of my adjustments lowers the height of an image's histogram, I suspect color quality is being lost as the pixel count is indicated by the height of the histogram above any tonal area. Is this proper thinking?

  2. #2
    DanK's Avatar
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    Re: Photoshop RAW Histogram Question

    If In understand your question correctly, the answer is no.

    The histogram reflects the distribution of luminance, not color quality.

    Start with the total histogram, gray on all of the cameras I have used. This represents total luminance, with all color channels counted. Say that you have a narrow, peaked histogram to start, but you move out the black and white points. This will widen the histogram and lower the peak. All this means is that the pixels have been redistributed among the various luminance levels, with fewer toward the middle and more toward the extremes. The count is decreased in one area and increased in others. This has nothing to do with color, or color quality.

    The slightly more complicated case is a histogram that shows all three color channels. Suppose you have a peak of the red histogram at the high end, say around 200. If you lower the luminance of the red channel, that peak will move left, giving you more red pixels at a lower luminance level and fewer around 200. The image will look less red, but the red is simply darker, not lower quality.

    This is a bit of a simplification, but I think the basic points are right.

  3. #3

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    Re: Photoshop RAW Histogram Question

    To add to Dan's helpful post, when the values indicated on the vertical axis of the histogram change, you have merely remapped some of the tonal values in the range of 0 - 255. You haven't changed the quality of any pixels for better or for worse.

  4. #4
    Administrator Manfred M's Avatar
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    Re: Photoshop RAW Histogram Question

    The other thing to remember is that your historgram (even when you are working with raw data), is still based on the jpeg. This means that there is a LOT more data that Photoshop can work with than is shown. If you are using a wider gamut colour space like AdobeRGB or better yet, ProPhoto, more of the colours that your camera has recorded are available to the editor.

    Moving the sliders to the left (setting your black point) and to the right (white point) redistribute the colours you have recorded. As Mike and Dan have alluded, this does not mean less colour intensity, but rather more distinct colours are shown. This usually gives you a munch more contrasty image with a much wider colour range.

    So, as Dan has already said, you are not losing anything, but rather are gaining the full range of colours, from the lightest to the darkest. I find that doing this virtually always gets you better colours, and is one of the first things that I do when editing an image.

  5. #5

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    Re: Photoshop RAW Histogram Question

    should keep ya busy for a few minutes...http://www.bing.com/videos/search?q=...gram&FORM=VDRE

  6. #6
    dje's Avatar
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    Re: Photoshop RAW Histogram Question

    Ed could I suggest that you have a close look at the two part CiC tutorial on histograms - it really is very good.

    The bottom line is that, as Dan indicates, histograms don't give you any information on perceived colour in an image.

    In PS proper, there are 6 histograms available

    RGB
    R
    G
    B
    Luminosity
    Colours

    In ACR there is only one histogram available which I believe is similar to Colours in PS.

    The individual colour channel histograms only tell you what is happening in individual channels and tell you nothing about how the channels combine in individual pixels to form colours. The RGB and Colours histograms just represent the sum of the individual R, G and B channel histograms and once again tell you nothing about how the channels combine in individual pixels to form colours.

    The Luminosity histogram is the only one that combines the three colour channels for each pixel but this only gives an indication of brightness with no colour information.

    Personally the only time I worry about histograms is when I'm looking at the top or bottom end for clipping and adjustment.

    Dave

  7. #7

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    Re: Photoshop RAW Histogram Question

    Thank you! I get it.

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