Hi Chris,
A quick and dirty "level set" using PhotoShop:
1. Open Levels dialog. The histogram shows relative frequencies of
occurrence of the 255 possible levels. Taller peaks=more pixels in the image
at that level. Make sure preview is ON.
2. Drag dark triangle on left to the leftmost point of the histogram (if the
histogram values and the triangle DO NOT coincide) or to the point on the
LEFT at which the first significant (i.e. more than a very small histogram
level) values occur (if the histogram values and the triangle DO coincide).
This sets the "black value". Any histogram values LEFT of this triangle
after you adjust will be "true black".
3. Drag open triangle at right of histogram to the rightmost point on the
histogram (if the histogram values and the triangle DO NOT coincide) or to
the point on the RIGHT at which the first significant (i.e. more than a very
small histogram level) values occur (if the histogram values and the
triangle DO coincide). This sets the "white value". Any histogram values
RIGHT of this triangle after you adjust will be "true white".
4. Adjust the gray triangle in the middle of the histogram to achieve a
decent midrange point. Just do it by eye until the picture looks good.
Keep an eye on your preview image as your making these settings, and you'll
see how things change. What you're doing here is "mapping" the (up to) 255
possible levels of the scanned image to the 255 possible output levels that
PhotoShop will use. Hopefully, this explanation will reduce the "trial and
error" nature of doing levels for you a bit.
Somebody more expert than I can confirm if this is overall "luminance" we're
setting. I believe the same sort of thing is possible for individual colors
in the image. I can't take credit for the explanation, either, I read it
somewhere. :-)
HTH!
---
Scott Gomez
-----Original Message-----
From: Chris Barker [mailto:imagopus@xxxxxxxxxxxxxxxx]
Subject: Re: [OM]
<snip>
A parallel argument is my use of Photoshop (Elements): I still don't
fully understand the use of levels and histograms, so my
effectiveness using trial and error is, I am sure, limited compared
to someone who does understand...
Chris
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