RE: Source for IT8 targets?

["Paul D. DeRocco" <pderocco@xxxxxxxxxxxxx>]

The data that accompanies the target indicates the colors of the piece of
film (or at least the average for that batch of IT8 targets), not the colors
of the original light used to expose the target film. So you would be
correct that any target should suffice for calibrating a scanner, if color
were really just a simple combination of three frequencies representing red,
green and blue. But light is made up of an infinite number of frequencies
covering a particular range, and all RGB devices (including the eye)
perceive color by applying three different spectral response curves to the
incoming light, one peaking at red, another at green, and another at blue.
Film itself has basically three colors of dyes that absorb red, green and
blue, but the precise spectral response curves of these dyes don't exactly
match the curves of the rod cells in the eye. Nor do they precisely match
the curves of the CCD elements in a scanner. Because of this, it's possible
for a scanner calibrated with an IT8 target made out of one type of film to
produce slightly inaccurate scans when scanning a different type of film.

But these are second-order effects. A scanner calibrated with any IT8 target
will produce good results on any type of film, just not perfect. As I said
earlier, if you want scans to look like the original scene, you'll have to
tweak the colors anyway, so it probably doesn't matter, but if you want
scans that look exactly like the slide, because you like the color rendition
of that type of film, then you should ideally calibrate with a matching
target.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

> From: gary
>
> I really don't follow this logic.
>
> Is the IT8 image done with a camera, light, and physical target, i.e. you
> photograph a standard of some sort? Then it makes sense to me
> that you could
> need such a target for each variant of film. If the IT8 by itself
> represents
> a standard for the scanner to scan, then the film shouldn't
> matter, i.e. the
> manufacturer of the target should compensate for the film.

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