[filmscanners] Re: Scanning negs vs. slides

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

Julian writes:

> My question:  The business of density ranges
> and recorded exposure range is, I thought,
> not a mystery at all if you look at the
> transfer curves.

Yup.

> ... in fact slides are recorded with a density
> range around 10 times that of the original scene
> - i.e. significantly more contrast.

Yes.

> I can only assume that people find slides
> more pleasing when the contrast is enhanced ...

Yes.  If you are projecting slides, chances are that you can reproduce only
a fraction of the dynamic range of the real world with your projector, so by
boosting the contrast in slides, you can reproduce more of the sharp
differences in luminosity of the real world with a limited projector--at
least over a small range of luminosities.  Your projector lamp isn't going
to be as bright as the sun, and the room your in probably won't be pitch
black, so exaggerating contrast will compensate partially for this and make
the image look more as it originally did.  The drawback to this, of course,
is that the film itself is limited in its range of densities, too, so
highlights will tend to turn completely white, and shadows will tend to turn
completely black.

Note that if you had a magic film with a perfectly linear response to light
and infinitely fine resolution of adjacent densities, you wouldn't need to
worry about any of this.  You could expose the film with any exposure you
pleased, then scan it, then expand the part of the image containing the
detail you want until it covered the full range of your display device.
Unfortunately, that's not possible.  And such a film would show
extraordinarily low contrast without such manipulation.

Another analogy is with color spaces.  The Wide Gamut RGB color space can
cover a huge range of colors, almost as many as the eye can see.  The
popular sRGB color space, in contrast, can cover only a very limited range
of colors, roughly comparable to the modest capabilities of things like
monitors and printed papers.  And yet the sRGB color space looks _nicer_
when displayed than the Wide Gamut RGB.  Why?  Because sRGB holds all its
image information within a restricted range that can be accepted by an
average display device, whereas Wide Gamut RGB spreads this information over
a huge range, much of which is beyond the capability of a typical
display--the small part of Wide Gamut that can actually be displayed looks
flat and washed out on a typical display, because so much of the information
capacity has been spent on undisplayable colors.



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