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filmscanners: Autoexposure problem in Vuescan

I've been scanning some airshow photos and getting the right exposure out of
vuescan has been extremely difficult.  The day was very overcast, and
there's a lot of photos where most of the frame is clouds with some very
small aircraft.  Vuescan seems to be using an autoexposure algorithm similar
to autolevels, but whatever the reason I end up with highlighted grain and
excessive contrast.

The most frustrating thing is that the preview image looks fine, but I can't
find a setting which makes the final image look anything like the preview.
Please refer to the attached jpeg for a comparison.  The jpeg is small but
it's big enough to see the huge difference between the preview and the final
image.  It's NOT just the cropping process which is the problem here - it's
the scanning exposure as well.  I've tried scanning from the raw file and I
can't get a good result with the data that has been scanned.

The only workaround I've found is to deliberately include part of the film
mask in the scan.  This presumably gives vuescan a "true black" to work
from.  If you crop back to just the sky, vuescan seems to try to expand the
grey values too much and you end up with an awful result.

I'm sure by juggling manual exposure values manual black and white points
etc it would be possible to get a good result, but I just don't have time to
do that for every frame.  I've tried white balance, neutral, etc but it's
all to no avail because the exposure was wrong before it got to the cropping

Any suggestions?  ISTM the only option is to do a preview which includes the
film mask, then switch to manual exposure, and keep that setting for the
rest of the film (or at least the strip).


Attachment: exposure_error.jpg
Description: JPEG image


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