P3D Re: Scanning slides automatically

["Greg Wageman" <gjw@xxxxxxxxxx>]

>This may have ben covered before, but has anyone set up the parameters
on a
>Nikon Coolscan to scan in Realist frames?


I've scanned quite a few slides now in my Coolscan II, and if I've
learned anything in the process, it's that a "one-size-fits-all"
approach doesn't work very well if you're looking for the optimum result
from every scan.

It's important to remember that the density range of a slide can vary
significantly depending on exposure and subject matter.  One way to look
at it is that not every slide is going to run the gamut from pure black
(max. density) to pure white (min. density).  The trick to getting a
good scan is to get the right mapping between the "useful" density range
in the slide and the entire range of the scanner's output.

My first thought was that I ought to be able to come up with a setting
for each film type that I use, and everything would be automatic after
that.  But that turned out not to be the case, for the reasons
mentioned: exposure variations and subject matter.  Through
trial-and-error, I developed a procedure which I think produces good
results consistently.

I always start with the RGB and master lookup table (LUT) ramps set to
their linear default.  I focus the slide (this may be an automatic
operation on some scanners, but it is manually initiated by a
button-press on the Coolscan II).  Make sure pre-scan is enabled, and
scan a preview image.  After the preview image is acquired, I look at
the histogram for the scan.  It is usually obvious when a portion of the
data range is sparsely represented, or not used at all.  Such scans will
benefit from further adjustment.  I hit the auto adjust button, which
lets the scanner software pick the LUT values based on the histogram,
and observe the change in the scanned image.  Often this is sufficient;
I do another preview image with the new values to check.  When I'm
satisfied, I disable the pre-scan function and acquire the final image.
I use the identical settings for the other view, except for focus.  I've
found that it's important to disable the pre-scan function for the other
eye's image; otherwise, the scanner may produce a different exposure in
spite of all else being equal.

Occasionally, the auto-adjust will be insufficient, or will skew color
values.  If the former, I reset the ramps to linear default, then use
the cropping tool to select a portion of the image that should be
"correctly exposed", i.e. is not shadows or highlights, then use the
auto-adjust on that.  This almost always works, as long as you choose
the right region.  If color skew is a problem, I adjust the RGB ramps by
hand to bring them back in line with 'reality'.

It's important to remember that the total density range of a color slide
can exceed the dynamic range of these relatively inexpensive desktop
scanners.  That's why some human judgment is often necessary to decide
which end, or ends, of the data to throw away.  "Wasting" pixel values
on indistinguishable shades of deep shadow leaves fewer pixel values
available for representing subtle changes in the useful tones of the
image.  This usually results in those visible quantization shifts and
false contourings which scream "digital image", and not in a good way.
Put in a qualitative way, this means that you may have to choose to let
your shadows go black or your highlights burn out (or both in extreme
cases), in order to get a smooth, natural representation in the
mid-tones.

     -Greg W. (gjw@xxxxxxxxxx)