Thanks, Bart, sorry for bothering
But, is any target profiling (apart
from VS own) finally based on a raw scan or on a tiff scan with no color
correction? How do I avoid any color correction in VS in order to provide
another profiling system with "untouched" data?
From what I've experienced, "scanning it right" is more difficult than
it seems.
1) Getting the right illumination.
This is not so trivial.
The target was meant to be scanned in "optimal light conditions"
(about 5500K, uniform, non-flaring). Some scanners have light
source-related problems, such as dark corners. If the scanning
software won't account for that (i.e., calibration), you could get
some patches darker than they should be, not because the scanner has
linearity issues but because of the dark corners...
2) Getting the right IT8 grid shape.
Often the target slide won't be perfectly "squared off". You'll have
to adjust the IT8 grid to have the measuring areas right on the
patches. Vuescan, for example, won't allow for any non-parallel
stretching (while LCMS profiler will allow any kind of stretching), so
you can't compensate for a slightly rotated slide, for example.
Yes, there's some tolerance 'cause only the centers of the areas are
measured, but still, if the film carrier is poorly designed, you can
have troubles having a good match.
3) Getting the right exposure
That's quite difficult.
Each scanner has its way to meter the exposure. The target has some
light (even nearly-white, but not *perfectly* transparent) patches and
some dark (even nearly-black, but not *fully* opaque) patches.
It's not trivial to have a good exposure, that is needed to have
"good" black patches and "good" white patches.
I found that often, I have troubles getting "good whites" and "good
blacks" at the same time. This affects profile building. I often scan
the target at various fixed exposures, try to have a good monitor
match (against the light table), select the "best" scans, and build
various profiles against those ones. Then, I check the profiles to
select the better one.
If you have an underexposed target scan, for example, the profile will
raise the luminosity of all your scans: not quite what you'd want.
4) Getting the right histogram.
The Color Balance setting in Vuescan is difficult to understand
completely, for me at least. What's the difference between "none" and
"neutral", for example? You could say it's the black and white points,
but it isn't: if you set both to 0%, still Neutral is different from
None.
"Neutral" seems to boost contrast a bit (even with both points at 0%),
and it seems to help having a better target scan (black and white
patches are "more black" and "more white", so nearer to the expected
reference IT8 values). But you see, it's not very clear how this
works...
5) Getting the right output color space, including gamma point and
white point (light source) setting.
Again, this is not trivial, for me at least.
We don't know exactly how the profiler that's embedded into Vuescan
works, but at least Vuescan knows it
; what about profiling with an
external tool like LCMS? Better to have the TIFF in AdobeRGB or in
sRGB? And does it matter at all? Better a 6500K white point as it's
the more widely used (and likely the one you'll process your images
with), or a 5000-5500K one (the one that the target vendor measured
with)?
Better a linear 1.0 gamma or a typical 2.2 one?
So you see, I'm quite confused too.
Let's hope we get enlighting answers by knowledgeable people (and
hopefully, from Ed Hamrick and Wolf Faust, too!).
All that said, I had good results with Vuescan profiles: better than
LCMS ones, probably 'cause with LCMS you have so many variables (from
scanning parameters to profile calculation parameters) that I mess it
up more easily.
Another big advantage of the matrix-based profiles that Vuescan
produces, is that they are tolerant to exposure variations.
This is agood thing, since I always adjust the CCD exposure to have
the optimal value when scanning slides (ie., if a slide is uniformly
dark, I boost the exposure in order to get the maximum detail from the
shadows, and then re-adjust the histograms within Photoshop).
Exposure varations are not well accounted for, with a LUT-based
profile ala LCMS.
Fernando
