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[filmscanners] Re: Density vs Dynamic range



Todd writes:

> ... if your luminosity range contains a 6 stop
> range, which is within the tolerance of slide
> film, and you shoot the scene on slide film and
> neg film, of the two frames of film I believe
> the slide frame would be the one with the higher
> dynamic range.

No passive system has dynamic range.  Slides and negatives do not, in
themselves, have dynamic range.  Only the combination of a slide or negative
with some sort of active process--such as viewing, scanning, exposure, or
development--can have dynamic range.  Slides and negatives have only density
ranges inherently, not dynamic ranges.  A piece of film can be opaque or
transparent or somewhere in between, and that's a range of density, but it
won't have any dynamic range until you shine some light through it.

> It would be taking the same luminosity, but
> spreading it across a larger density range
> than neg film, without clipping, and with the
> same noise.

Quite so ... and this is why slides will tend to look better for photographs
that fit withing their usable exposure range.  The difference in density
produced for a given change in luminosity in the original scene is greater
for slides than for negatives, so as long as a given part of a scene falls
within the density range of a slide, the slide will produce more tonal
resolution.  A corollary of this is that slides will show more contrast.
This higher contrast and greater tonal resolution more closely resemble our
normal visual perception, and so can accentuate the realism of a rendered
scene.

The problem is that neither negatives nor slides have unlimited density
ranges.  Slides use up what they have over a fairly narrow range of
luminosities, making the rendering of that narrow range very realistic and
impressive, but limited what can be rendered outside the range (in
highlights and shadows).  Negatives spread their usable density over a wider
range of luminosities, allow greater extremes of luminosity in the original
scene to be usably recorded, but considerably diminishing the contrast,
tonal resolution, and realism of midtones in their rendering of a scene.

In many cases, it may be hard to tell the difference, especially when
scanning.  I think that, for scenes that do not contain important extremes
of luminosity, slide film still has an edge, if you have a scanner that can
extract everything from the slides.

> The neg film would be compressing the relatively
> short luminosity into a lower density range.

Exactly.

> IOW, once the film exists as a photo, and the
> luminosity or the original scene is no longer
> relevant, the dynamic ranges of the two pieces
> of film itself would be different than that
> of the capability of the films under
> different original conditions.

Yes.  Dynamic range is a function of an active system, not a passive piece
of film.  If you don't shine any light through a slide, its "dynamic range"
is zero.

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