Levels white dropper makes bright point yellow?

Discussion in 'Photoshop Tutorials' started by c-bee1, Jun 10, 2004.

  1. c-bee1

    c-bee1 Guest

    Hi all. Our photoshop at work is doing something to one image that it
    doesn't do to others.

    We use the 'white point set dropper' in 'adjust levels' in order to change
    the 'white balance' of an image for on-the-fly camera illumination
    correction on a research microscope.

    Basically, we shoot a featureless 'background' image on the scope (which
    looks like an off-white frame that is brighter in the middle, but not white,
    and fades toward the edges), then move it into photoshop, and after a
    gaussian blur, we white-point-set it at the brightest central point, at
    which time the middle of the image turns white, and the edges fade out to
    the real color of the illumination impurity because of the math..

    Then we reload that into the camera, and our subsequent images are evenly
    illuminated using that correction. But that's not the problem -

    This one lady is shooting a gold film surface that is coppery brown in
    color, to get her illumination frame, which is then coppery in the middle
    and fading to brown at the edges. When we use the white point set dropper
    in the middle of this, it doesn't turn the middle white, like every other
    image we've done - it turns it pure yellow.

    Any idea what is happening, and how we can fix it? We need this coppery
    image adjusted so the brightest point is white, and fading toward the
    edges - if we don't we will get an anti-yellow image (cyan?) from the
    camera! Is this explanation making sense?

    Any help vastly appreciated! -cmb, microscopy guy
     
    c-bee1, Jun 10, 2004
    #1
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  2. c-bee1

    eastern Guest

    Of hand, absent more information, I'd say it's because your eyedropper
    sampling setting (probably something other than 1x1) results in an average
    such that none of the individual pixels in the selected area goes to white
    when you use the white eyedropper.

    Dane
     
    eastern, Jun 10, 2004
    #2
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  3. c-bee1

    eastern Guest

    Off hand, absent more information, I'd say it's because your eyedropper
    sampling setting (probably something other than 1x1) results in an average
    such that none of the individual pixels in the selected area goes to white
    when you use the white eyedropper.

    Dane
     
    eastern, Jun 10, 2004
    #3
  4. c-bee1

    c-bee1 Guest

    Thanks, guy, great answer - I didn't know there was a sampling size
    control, and I will try that (I'm home sick today). But it leaves me
    wondering, for the following reasons:

    One, that in this area, the pixels are all very very close to each other
    in value, certainly not diverging by more than 1 or 2 in R,G,or B, and we
    could certainly deal with a resulting 'white' that was off by that much.
    (to imagine the picture we are sampling, think of a single copper-colored
    light bulb a couple feet behind frosted glass - the big fuzzy spot is almost
    uniformly 'bulb-colored' in the middle), and

    Two, is that upon further thought, what the computer seems to be doing is
    expanding red and green to 255 like one would expect, and completely
    ignoring the blue level - resulting in that perfect lemon yellow we all
    know.

    So I guess what I'm wondering is, what would make the white point dropper
    ignore the blue level when it expands the histogram? (hoping my terminology
    is correct here)...Certainly there is almost no blue in this coppery field
    we are starting with - maybe I need to goose it a tiny bit in
    'variations'..? I would think that 'set white point' would drag all three
    colors to 255 like it usually does for us, no matter what we started with.

    Just thinking out loud here.
     
    c-bee1, Jun 10, 2004
    #4
  5. Double-click on the highlight dropper in levels and make sure that the
    colorpicker is set to white.
     
    Scott Southerland, Jun 10, 2004
    #5
  6. c-bee1

    eastern Guest

    Despite the received notion that each channel is forced to 255 what really
    happens is that the white eyedropper multiplies by 16 and truncates to 255.
    Thus if a channel value is very low, e.g. blue=5, you'll get a non-white
    value. That's another reason for no change.

    Personally, I find the eyedropper tools to find either of the BW points to
    be too unpredictable to be useful. Instead I use the sliders in the levels
    and curves to set the BW points.

    Dane
     
    eastern, Jun 10, 2004
    #6
  7. c-bee1

    -xiray- Guest

    My experience as well. Rather than set any color using the eyedropper
    a better procedure would be to use the Adjust | Curves dialog box.
    This takes the guess work out of trying to select a particular pixel
    on screen with the eyedropper.

    And as a more complex issue, if your photomicrographs are destined to
    be printed on paper (like in a book or journal article) there are some
    considerations that your publisher will have concerning the brightest
    white and deepest color saturation areas of your images. But that's
    another story entirely.
     
    -xiray-, Jun 14, 2004
    #7
  8. c-bee1

    c-bee1 Guest

    Ah! I get it now. That is indeed the case in our trouble images.
    I have found that temporarily sliding the left triangle up to get more
    contrast often shows up the brightest areas.
    I'll have to try that. My ideal situation is to have a procedure that an
    unPStalented microcope user can read off of a page. Luckily, so far the
    saving grace has been that at the center, pixel values are pretty close to
    each other and + or - say, 10, is not a real problem.
     
    c-bee1, Jun 17, 2004
    #8
  9. c-bee1

    c-bee1 Guest

    Hey! Thanks, I'll try that.
     
    c-bee1, Jun 17, 2004
    #9
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