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[filmscanners] Re: vuescan: IT8 calibration for negatives?



simpy wrote:

> 1) I usually use negative film. Does IT8 calibration make a difference
> for negatives as well? I can imagine that the difference will be less
> obvious than for slides, because the color-cast removal messes up the
> colors anyway. On the other hand, a calibrated scanner could give a
> better starting position for vuescan to remove the color cast...

Vuescan bases its knowledge of each device's (scanner's) colourspace on the
IT8 Q60. This is generic and individual scanners of the same model will
vary a little, but nothing like as much as film types. The point is,
though, is that what needs to be applied to the raw data is a composite of
scanner+film type.

Where colour neg is concerned, there are too many variables for film
characterisation to  get you very far. Exposure, colour temp. and
processing each have a much wider range than slide films, and consequently
there's too a wide range for profiling to be any help. Besides, with slide
you'll generally be aiming at a scan that is the best approximation to
what's on the film. This isn't possible with  neg, where almost everything
is a matter of interpretation and judgement/intent. There's a 3-D magnitude
of freedom which just doesn't apply to slides, but that's partly  why many
choose to use it. Having got all that extra information onto film,
there's often far more than will make a satisfactory picture without
working on levels and curves.  Hue and luminance correction by profile
would be a straitjacket, even if it did work.

> 2) I found that you can usually buy different types of IT8 reference
> targets; one for every major type of film (ektachrome/velvia/other
> fuji...). Why would you need a different target for every type of film?
> I would assume that there is only one `correct' calibration, regardless
> of the film material. What am I missing?

As mentioned, what needs to be applied to the raw data is a profile that
characterises the scanner's response to coloured dyes  of different
densities. The trouble is that different films use different dye sets which
the scanner sees differently, and not necessarily in the same way that the
eye works. Two different dye images that appear similar to the eye may
'look' completely different to the scanner's different chromatic response -
 which extends into IR, and often UV as well, I believe. So each mfr.
produces an IT80 which uses their dyes, and each requires a different
profile.

> 3) IF IT8 calibration is useful for negatives and there IS a difference
> between target types, which one would be the best?

It isn't, none are, and AFAIK no IT8's on colour neg exist. Really you have
 to approach each colour neg image (or at least subject) as a new colour
correction challenge. You can make this easier by photographing a greycard
a white card and a black card, for each set of subject conditions you
encounter on each roll. Sometimes that will give a good basis for colour
correction using the PS levels eyedroppers. But other times it won't, not
least because you don't want the result to be quite like the original
scene anyway. EG neutralise the warmth in a sunset, and it no longer
looks like a sunset anymore but a mistake. Our eye/brain is subtler than a
colorimeter, or profiles.


Regards

Tony Sleep - http://www.halftone.co.uk
Online portfolio & exhibit + film scanner info & comparisons
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