How do I find neutral gray point - help?

Discussion in 'Photoshop Tutorials' started by Lynn, Jun 10, 2004.

  1. Lynn

    Lynn Guest

    Hi, I am occasionally having colour cast problems with some of my
    photos and I know I should be finding a neutral gray point on my photo
    and then using the grey eyedropper in levels to help. Problem is, I
    can't seem to FIND neautral gray in color images.
    I can get the blackest and whitest points using threshold and am
    wondering if there's a similar way of finding grey?
    I'm sure many people can simply look at a photo and see where gray is,
    but I, sadly, don't seem to be one of them. If there's a way in PS CS
    to find the grey, I would SO love to know what it is. Thanks so much
    for all ideas! Lynn
     
    Lynn, Jun 10, 2004
    #1
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  2. Lynn

    L. McKenzie Guest

    Look for a color in your photo that looks as close to gray as you can
    find. Click the midpoint eyedropper on that color. I recommend using
    Curves instead of Levels for most operations due to the fact that you
    can uniquely control all 255 shades of gray. If you don't have ANY gray
    in your photo, do a test shot using a calibrated Gray Card Wedge Step (A
    card calibrated with pure gray color on it). Use that card in your test
    photo, remove the card for your actual photos, you can calibrate color
    and remove color cast from the first.
     
    L. McKenzie, Jun 10, 2004
    #2
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  3. Lynn

    Joyp Guest

    do a test shot using a calibrated Gray Card Wedge

    Where does one find a calibrated Gray Card Wedge
     
    Joyp, Jun 18, 2004
    #3
  4. Lynn

    one_of_many Guest

    one_of_many, Jun 18, 2004
    #4
  5. Lynn

    Mike Russell Guest

    I'd like to chime in once more with a different tack on selecting a neutral
    gray point in an image. After selecting a highlight and shadow, getting rid
    of any overall color cast is the most important color correction operation.

    The term "neutral gray point" is normally used in color correction, and in
    Photoshop selecting a gray point can be done by placing an eyedropper point
    on an object in the image that is known to be gray. Then use curves to make
    the red, green, and blue values equal.

    It's very important that the object be one that is truly gray, and not
    something that is suspected gray. For example, an article of clothing may
    be ivory or light blue. As someone else mentioned, the lower edges of
    clouds are normally a perfect gray color. Sidewalks, asphalt streets,
    kitchen appliances, sheets of newsprint, brushed aluminum or steel, iron, a
    bar of ivory soap, etc. The list goes on, and it is based on our common
    knowledge of the world, or our memory of the scene.

    I have to confess to sometimes picking a variety of "gray" objects, and
    setting on an object that, when neutralized, makes the rest of the image
    look good. This is particularly easy to do using the Curvemeister plugin in
    Lab mode, which allows you to drag the neutral around and see how the colors
    change. The gray eyedropper in Photoshop curves behaves in a similar way,
    but is disabled in Lab mode, which is my favorite color space for finding
    neutrals.

    Some images - roughly speaking one in 20 - have two light sources. An
    example would be a person standing near a sunlit window. The interor
    lighting is normally colored by the objects and walls of the room, or
    artificial lighting, and will create a different color cast than the light
    from the window. These are difficult images to deal with, and in extreme
    cases may require that a mask be used to separately color correct the
    different parts of the image. Luckily such images are rare.

    IMHO, a gray card or other calibrated card in the scene is usually not
    helpful, and can be very misleading in mixed lighting situations. Calibrated
    targets can be invaluable for experimentation, or in a studio setting where
    lighting consistency is crucial. Examples would be quantitative scientific
    work, professional portraiture, or catalog work where a large number of
    images need to match one another.

    There is additional material about neutral points, plus a free demo with
    tutorials that include setting the neutral point, at the Curvemeister site.
     
    Mike Russell, Jun 18, 2004
    #5
  6. Lynn

    nikki Guest

    A note on using the Eyedropper. I read somewhere that whatever Grid
    size(Single pixel, 3x3 grid avg., or 5x5 grid avg.) you select when
    using the Color Picker Tool, that size stays with all your color
    picking from that point on.
    Make sure you have it set at 3x3 average or even 5x5 average, and not
    on Single Pixel color picking. If you want to see the variety of
    colors in a Grey background, zoom in to Pixel level magnification. If
    you average the colors in a 3 or 5 square grid, you have a better
    chance of hitting a grey average.
    nikki
     
    nikki, Jun 19, 2004
    #6
  7. Lynn

    savecurve Guest

    Very valid points and excellent suggestions. Most books and tutorials
    will teach you how to *set* black/white/gray points, but never teach you
    how to *identify* them first. Nor do they teach you how to handle images
    where there are *no* true neutrals to begin with, e.g. a sunrise/sunset
    landscape scene. Dan Margulis' book is an exception.

    With this awareness, I still have questions.

    I normally work in the RGB mode. Why is LAB mode special in finding
    neutrals?
    You may be correct that only ~5% of the images have two or more
    (direct?) light sources. But in addition to being lit by these couple of
    (direct?) light sources, an object in an image is also lit by indirect
    and reflected light sources. It is not always necessary to correct the
    multiple light sources. When it is, I find applying curve based on these
    objects can mess up the whole image, and masking these objects is one
    way out.
    I agree that a gray card is not appropriate when there are more light
    sources than you can account for.
     
    savecurve, Jun 19, 2004
    #7
  8. Lynn

    Mike Russell Guest

    Yes. Dan's the man!
    [re setting or (sometimes) searching for a neutral]
    The main reason is you can set the neutral with no effect on the image's
    overall brightness. In Photoshop this may be done manually to zero-ing out
    the a and b values for a particular color,and in Curvemeister you can just
    drag the neutral point around.

    [re images with mixed lighting]
    Absolutely. I almost never mask - but there are occasional images where it
    is necessary.
    Inserting an object into your images is really not practical except for
    posed pictures, or in a studio setting. If I were doing a set of wedding
    pictures, say, I might do a few frames of Macbeth checker just for
    insurance. In the days of film, this was a good way to tell if your film
    had been developed and printed correctily, and if not why not. For
    experimenting in Photoshop a piece of newspaper, the back of a shirt
    cardboard, or a page from a magazine will do in a pinch.

    Standardized gray cards are another matter. Those puppies are not cheap.
    There are some pretty severe requiremenst for reflective standards,
    including stability and spectral distribution, which is non-trivial for
    pigment based colors. A Macbeth card is about $90, and a gray card is
    probably in the neighborhood of $30 to $50.
     
    Mike Russell, Jun 19, 2004
    #8
  9. Lynn

    savecurve Guest

    Setting and correcting w/b/gray points is a great way to fix an image,
    *provided* that these points do exist in the image *and* you know how to
    identifying them. All the books and tutorials will use examples that
    contain real w/b/gray points which are easy to identify. And they work
    great. But in the real world, as discussed above, in many images these
    points are difficult to identify or simply do not exist. Correcting this
    kind of images right really separates the men from the boys (or women
    from the girls).

    Several color casts typically exists in these difficult images and there
    are no w/b/gray points. I want to preserve the desirable cast(s)
    (sunrise/sunset glow, candle light indoors, etc.), but at the same time
    remove the undesirable cast(s). If I apply curves to channels
    iteratively to remove the casts, my lying eyes and short memory cannot
    keep track of whether one edit is "better" than another. What I need is
    a way to compare the edits side by side. I find PS' Variations to be
    rather crude for this purpose, and duplicating the image in multiple
    windows to be tedious.
     
    savecurve, Jun 22, 2004
    #9
  10. Lynn

    Mike Russell Guest

    Good point. Check out the Compare button in the Curvemeister demo - it will
    toggle between the corrected and uncorrected images, and you can also
    quickly switch between color spaces and compare your work in each color
    space as well.
     
    Mike Russell, Jun 22, 2004
    #10
  11. Lynn

    Tom Nelson Guest

    To the other excellent responses I'll merely add the following:

    1. Your perception of a color cast is most acute in the lighter values.
    Neutralizing the white point goes a long way toward getting a pleasing
    (as opposed to objectively neutral) color balance.

    2. Don't forget to look at shadows on known white objects. These may
    likely be middle gray. Keep in mind that outdoor shadows are blueish -
    try try going halfway toward neutral with them (using the RGB numbers
    in the Info palette) or equalize the R and G numbers.

    3. Without a known white or gray object in the scene, the viewer will
    have as much difficulty as you in detecting a color shift.

    4. In people photography, viewers have a wide tolerance for warm
    (yellow-to-red) color shifts.

    Tom Nelson
    Tom Nelson Photography
     
    Tom Nelson, Jun 22, 2004
    #11
  12. Lynn

    savecurve Guest

    These are very helpful points to keep in mind when dealing with neutrals
    (or lack of). Just like neutrals, there are recommended color values for
    the different skin tones. But remember that these values are for the
    skin tones *without* any cast(s). They need to be corrected just like
    correcting neutrals, i.e. taking desirable and undesirable cast(s) into
    consideration. Since most viewers are more familiar with or critical
    about skin tones, getting them right is even more important than
    neutrals.
     
    savecurve, Jun 23, 2004
    #12
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