[filmscanners] Re: Scanning negs vs. slides

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

Austin writes:

> Well, yes they do eliminate it.  Because of
> their exposure latitude and limited dynamic
> range.  The exposure latitude clips the ends.

And expands the midrange.  So you are trading one type of information for
another, but the total amount of image information on the slide is the same.

> That's not true.  Take a picture of a step
> wedge and find out for your self.

It cannot be otherwise, given the response curve of slide films, and the
fact that film is a continuous-tone medium.  Since the curve is steeper for
midtones than that of negative films, the difference between adjacent tones
in terms of film density in that midtone range is much greater on slides
than on negatives, thus giving slides more resolution in that area.  Of
course, since the curve is so steep, it bottoms out and tops out more
quickly, and so shadow and highlight detail is much more readily lost.

> Wrong.  The response curve has NOTHING to do
> with representing the dynamic range/ability
> to discern tones.

Sure it does.  It shows film density as a function of light exposure.  A
steep curve means that the density varies very rapidly with light exposure,
producing a lot of resolution of adjacent tones, but potentially poor
rendition of highlight and shadow (because most of the available density
range is already being used to represent the midtones).  A shallow curve
means that adjacent tones are less clearly distinguishable, since they
produce less variation in density, but it also implies that highlights and
shadows will be more detailed, since more of the remaining density is
available to represent them.

And the whole purpose of the steep curves of slide film, as I've indicated,
is to better reproduce the dynamic range of a scene during projection.  The
midtones of a slide can be projected with differences that much more closely
approach those of the original scene; the drawback is that, since film
density is limited in its range, highlights and shadows tend to lose detail.

> Absolutely not true.  The exposure latitude for
> positive film is far less than for negative film,
> and given that, there is less scene information
> recorded on slide film.

Latitude is irrelevant.  The information-carrying capacity of the film is a
function of its density range, from clear film to the darkest possible
coloration of the emulsion.  Slide film has a greater density range than
negative film in this respect, and if we assume that both types of film
provide the same resolution between adjacent density levels (a reasonable
assumption), then slide film actually carries potentially more information
than negative film.  It is distributed differently, however.

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