Best RGB value of 'mid-grey' ?

Discussion in 'Digital Photography' started by Sorby, May 17, 2004.

  1. Sorby

    Sorby Guest

    My local printers print 10"x8" and 12"x10" but I need a 10"x10".
    So I have widened the original square-format photo so there is a 1" strip
    either side.
    These strips are both white but it occured to me this may skew the
    auto-colour correction when I get the lab to print them.
    Should I change the colour of these white strips to a mid-grey?
    If so, what is the value of mid-grey in RGB terms?

    Thanks
     
    Sorby, May 17, 2004
    #1
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  2. Sorby

    nixjunk Guest

    My local printers print 10"x8" and 12"x10" but I need a 10"x10".
    There should be no default correction unless you ask for it for digital images.
    Check to be sure. The only reason you should change it to a grey is to make it
    easier to see where to cut in case the area in the picture along the border is
    white or black.
     
    nixjunk, May 17, 2004
    #2
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  3. In RGB terms it is 128,128,128.
    In photographic 18% average reflection terms it is 117.

    Bart
     
    Bart van der Wolf, May 17, 2004
    #3
  4. []
    Assuming what value for gamma?

    David
     
    David J Taylor, May 17, 2004
    #4
  5. 2.2 as in sRGB, Adobe RGB or whatever most digicams use when creating a
    JPEG.

    I've given a more detailed explanation in an earlier, somewhat related,
    thread called "D70 Photoshop and gray cards".
    Since the OP already has a Gamma adjusted image he wants to add info to, 128
    is good enough (still depending on what the Lab does to the data).

    Bart
     
    Bart van der Wolf, May 17, 2004
    #5
  6. Thanks, I haven't done the sums to see if you are correct! Yes, 117
    sounds good to me for an 18% reflectance.

    Cheers,
    David
     
    David J Taylor, May 17, 2004
    #6
  7. Sorby

    Tony Spadaro Guest

    A Kodak grey card will be roughly 160 RG&B for a 2.2 Gamma CCalibrated
    monitor. I don't see why you need the background to be grey though. You can
    use anything you like and yes colour of the rebate will affect your
    perception of the colours in the photo, but they have no "physical" effect
    on them. Once you trim to the picture area all will be equal. Of course you
    could use the rebate to "matte" the picture. When I have pictures with white
    areas at or very near the edge I put a few pixel wide black border on the
    picture. IF the edge is black I use white and if there is black and white
    both to deal with I have a nice dark red, etc.

    --
    http://www.chapelhillnoir.com
    home of The Camera-ist's Manifesto
    The Improved Links Pages are at
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    A sample chapter from my novel "Haight-Ashbury" is at
    http://www.chapelhillnoir.com/writ/hait/hatitl.html
     
    Tony Spadaro, May 17, 2004
    #7
  8. It also depends on the image exposure. Does 255 represent the
    brightness of a mythical 100% reflectance object, or a more typical
    piece of white paper/cloth (90% reflectance), or did you allow more
    headroom for specular reflections and place white at 200 or 230 instead
    of 255?

    If you assume that 255 is 100% reflectance matte white, then 18%
    reflectance is 0.18^(1/2.2) * 255 = 0.459 * 255 = 117. But other
    exposure conditions yield different mid-grey values.

    Dave
     
    Dave Martindale, May 19, 2004
    #8
  9. Yes, and the 2.2 is certainly another element which can be different
    according to the actual display - i.e. what gamma you should apply when
    going from linear sensor space to gamma-corrected space. Should you apply
    the theoretical gamma and require a gamma 2.2 display (which is what I
    believe you should do) or adjust the gamma in the camera (its so-called
    "contrast" control) to suit your own display?

    The number of ways you can get things wrong without really trying!

    Cheers,
    David

    Cheers,
    David
     
    David J Taylor, May 19, 2004
    #9
  10. In this case, I'd say that doesn't matter. The thread is about what
    number you should put into an image *file* to represent mid-grey, and
    that depends on the standard gamma of the *file*. If the file uses
    sRGB encoding (and that's probably the default if there's no ICC info),
    then the file's gamma is 1/2.2, and that's the end of the matter.

    Now, if the display has a highly nonstandard gamma, correcting that is
    the job of the lookup table in the video card, or the display software.
    Thus a 117 pixel code in the file might end up as 110 or 125 going into
    the video DAC - but that's independent of what's in the file.

    (And, in fact, most CRTs have a gamma of 2.5 or higher, while the data
    in the file is 1/2.2, but no correction is done to reconcile the
    difference. In fact, this is deliberate, because it gives a slight
    contrast boost that most viewers prefer to completely accurate tonal
    reproduction when the display is viewed in a dim surround. So
    complete physical accuracy generally isn't desired!).

    You've also got to be awfully careful with controls called "contrast".
    In photography, and probably in digital cameras, a "contrast" control
    really adjusts contrast (gamma). In video, "contrast" is really video
    gain which adjusts what a photographer would call brightness. Meanwhile
    the video "brightness" control is really black level.

    Dave
     
    Dave Martindale, May 19, 2004
    #10
  11. []
    One of these days I will check my display. I have a special test card I
    designed with the two extremes of the brightness scale (0 and 255) having
    difference steps up to 32. So at the black end, there is a black region,
    with boxed text first at a brightness level of 1, then 2 etc. up to 32.
    At the white end, a background of 255, then text at 254, 253 etc. The
    idea is to adjust your monitor correctly, and see which is are the
    first-visible and last-visible patches. The program is available at:

    http://www.david-taylor.myby.co.uk/software/imaging.html#GreyScale

    For some reason, LCD displays seem to a little better at this than CRT
    displays - almost as if they have some sort of dynamic region contrast
    boost. I've never been able to explain it properly.
    Tell me about it! I hear people say: brightness adjusts the bright parts!
    Arrgh!!
    I haven't yet heard a [digital] photographer say they are going to adjust
    the gamma!

    Cheers,
    David
     
    David J Taylor, May 19, 2004
    #11
  12. You might hear me say that, if I thought you understood what it meant.
    Gamma *is* apparent contrast or, more precisely, the exponent when the
    transfer function of a system has a power-law shape. Film has gamma
    as does printing paper, and the gamma of a final image is the product
    of the gammas of all of the stages along the way. In digital
    photography, moving the middle slider in Photoshop's Levels dialog is
    adjusting image gamma.

    But to a photographer, contrast and gamma are pretty much
    interchangeable, so I tend to use contrast as long as it's clear that
    I'm not talking about video.

    Dave
     
    Dave Martindale, May 19, 2004
    #12
  13. Yes, so many times when you have to think: what background and expertise
    does this person have, so that I can explain something at the right
    level.....?

    Where analog and digital differ, though, is at the extremes of the
    transfer range, where the gamma approximation breaks down. With analog
    [film] there is just a little more detail in the shadows and highlights,
    if you can process to capture it.

    Cheers,
    David
     
    David J Taylor, May 20, 2004
    #13
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