[filmscanners] Re: understanding contaminated blacks--SS4000--newbietype question

["Anthony Atkielski" <anthony@atkielski.com>]

Stan writes:

> One fundamental problem is that my choice of
> films back when this was taken--Velvia--was
> quite inappropriate for this particular scene.

Velvia is probably the most challenging color film to scan; if your scanner
can handle Velvia, it can handle anything.  It looks great when you use it
for the types of scenes for which it was intended, but if you accidentally
use it for something else, it can cause grief in the digital lab.

> Conceptually, how is the green shift in
> underexposed areas different than the so-called
> reciprocity failure color shift seen with very
> long, low light exposures?

I don't think there's a relationship.

However, I do think the shift might have to do with underexposure.  The
shadow areas in an image are areas where the film is badly
underexposed--that's why they are shadows.  They are underexposed in the
sense that the film is not optimally activated by light, because there isn't
enough of it.  The color balance for most films is calculated for a correct
exposure of a neutral gray; for shadow and highlight areas, where exposure
might differ from this ideal by several stops, some color shifts can occur.
I think that the problems in shadows may be color shifts caused by the
extreme underexposure of those shadows.

> The Ektachome 400 films I have used for moonlight
> exposures tend to go magenta rather than green.
> Will Velvia show this green shift under those
> circumstances?

I don't remember how Velvia shifts for long exposures, but the shift is
significant (it's in the data sheets).  For night shots, I suggest Provia,
which has no reciprocity failure for exposures of up to two minutes.  Velvia
is too contrasty for night, anyway.

Most of the films I can remember using offhand show a green shift in
shadows, but I haven't tried Ektachrome.  Portra shows a lot of shift in
this regard; Portra 800, like all the Portra films, scans beautifully, but
when it is underexposed the noise in the green channel gets really
obstrusive.  Provia shifts in the same way, and Velvia certainly does.




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