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[filmscanners] Re: LS2000 Shadow Detail problem
>While doing scans of a transparency, I ran across a curious problem with
>the Nikon LS2000 losing shadow detail that was captured in the scan, but
>not retained when color management was used.
>I've detailed it with pictures and descriptions of my methodology at:
In addition to some of the good suggestions from folks on this list, here
is something else which seems to be involved with this problem.
Thor Olson, a color imaging scientist whom I am corresponding with
privately, also suggested that gamut mapping may be playing a part in this
problem. He told me some very interesting information...
>I don't think it is an intrinsic issue about color spaces;
>it's probably one about gamut mapping between different ones.
>Gamut mapping is still a very active research topic. One
>thing they've learned is that the best mapping is application
>and image-dependent, so it would not be surprising if the
>general purpose mappings performed by Photoshop do not meet
>the needs of all pictures!
>Most monitor spaces use an encoding for lightness that has a linear
>absolute zero, which then splices into a power law curve for the
>above around 8%. This is true for both sRGB and L*, and probably your other
>color spaces you tried. The deep shadow detail near D=3, would all get
>into a few counts of digital range.
>A decent film scanner would be loathe to lose this information in this
>way. It would
>probably use an encoding that was more like a power law all the way to
>the shadow detail across a larger number of counts.
>Of course when you try to display such an image, it will get mapped to the
>L* or sRGB
>curve with the linear segment, and the detail will be lost from view (but
>still encoded in
>the image of course).
>When you open the image with no profile conversion, PS treats the encoded
>if they were sRGB (even though the aren't) and the detail becomes
>visible. You have
>told PS to consider those encoded levels as being at a higher luminance
>than they were
>originally encoded at. It will show the shadows, but of course the tones
>(and colors) are
This seems to be the exact behavior I am getting. The pixel values for the
deepest shadow detail do not change with a profile assignment, but the
detail disappears visually. Curves cannot bring it back up either.
This is turning out to be a lot more complicated than I thought.
I don't know why I'm surprised. <G>
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