Auto Levels vs. Levels -- Histogram smoothness

Discussion in 'Photoshop Tutorials' started by Norm Dresner, Feb 14, 2008.

  1. Norm Dresner

    Norm Dresner Guest

    If I take an image whose brightness range doesn't span black to white and
    apply the Auto Levels command, I get a histogram that runs the entire range
    and is smooth, but if I do a supposedly identical thing with the Levels
    dialog box, I get a histogram that has many holes in it because the
    brightness levels are simply spread out and not interpolated.

    Is there any way to get the smooth histogram effect while using the Levels
    Dialog so I can manipulate the mid-grey level independently of the black and

    Norm Dresner, Feb 14, 2008
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  2. Norm Dresner

    Mike Russell Guest

    What you are seeing is the effect of auto levels modifying the individual
    RGB channels, then recombining them to calculate a composite histogram. If
    you look at the individual channels, you'll see the same combing (which is
    not normally a problem, by the way)
    Modify the individual channels, as the auto levels command does.
    Interpolating is not mathematically possible. Since the original color
    values are identical to one another, there is nothing to interpolate. You
    can get the effect you are after if you add noise to the final result.

    This is not part of your question, but it is better to use curves than
    levels. Curves are more flexible in every way than levels. The histogram
    combing and spiking is often cited as evidence of a problem, when it is not
    a problem at all. It is the natural result of rescaling a range of integer
    values. If you get visible banding in the image, (not just histogram
    combing), add a small amount of noise to the affected area.
    Mike Russell, Feb 14, 2008
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  3. Here's something to try with the Levels dialog box. Hold down "Alt" on the
    keyboard and click on the white highlight slider, your picture turns black,
    still holding Alt drag the white slider towards the middle, when you see a
    light pixel appear on your picture that's your brightest highlights so stop
    and leave the slider there.
    Do the same at the other end for your shadows.
    /\\BratMan/\\, Feb 14, 2008
  4. Norm Dresner

    KatWoman Guest

    auto levels and auto color is not reliable, if you like warm light, sunset
    shots etc
    it will make it hideous
    it is like getting a machine print instead of a custom print
    if you take pictures of a winter scene or underwater, for example, it will
    be a lot of highlights and bluish
    maybe you want to keep it
    some computer decides your image by numbers of what an AVERAGE shot should

    use your eyes on manual adjustments or the droppers

    or learn curves, a little harder at first but gives superior results

    listen to Mike Russell
    he is the curves expert here and has a webpage devoted to it
    KatWoman, Feb 14, 2008
  5. Norm Dresner

    Avery Guest

    but do it with the individual RGB channels, then adjust the center slider in each channel to get rid
    of color casts
    Avery, Feb 14, 2008
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