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[filmscanners] Re: Scanning negs vs. slides

Tony writes:

> With slide you discard a lot of image information
> at the shooting stage, with colour neg you defer
> those decisions until working on the scan ...

Not true.  Slides compress image information; they do not eliminate it.  In
a slide, you get extremely detailed gradations of tone from one zone to
another in the image, but the extremes of white and black are closer
together; in a negative, you get far less precision in the gradations of
tone, but the extremes of white and black are further apart.  Thus,
negatives will cover a wide range of luminosities, but with low contrast and
not much detail separate like tones in the image; slides will cover a narrow
range of luminosities, but with high (i.e., natural-looking) contrast and
very detailed gradations of tone.

This is easy to see just by looking at the response curves of these films.
Negative film has a broader, flatter response curve than slide film.  Both
types of film provide an equal amount of image information, but negative
film spreads it all over a wider range of luminosities.

Ideally, then, you would use negative film for extremely contrasty subjects
if you must retain more detail in highlights and shadows, but you'd use
slide film for low-contrast subjects in which very fine detail between
adjacent tones must be retained.  This would seem to suggest slides for
studio portraits and many outdoor shots on overcast days or away from bright
sunlight, but negative film for high-contrast, sunny outdoor subjects, night
shots (which tend to be high contrast), and things like shows where there is
a lot of high contrast in the subjects.

> Slide often forces you to sacrifice either
> shadow and/or highlight detail.

Yes, because they pack all that detail into the midrange.  So you get very
subtle gradations between two similar tones in the photo, but the highlights
tend to all turn white, and the shadows tend to all turn black.

This is because slides have to be projected.  In order to provide contrast
similar to the original, real-life subject, there has to be a lot of
contrast in the slide.  In reality, five stops is already 32:1, and eight
stops is 256:1; in order to show that on a projection, you need quite a
steep change in density on the film.

> With neg, you can if you wish retain both, by
> combining (say) an image which has good shadows
> and midtone separation but blown highlights,
> with one where you mask off the image apart
> from the highlights then adjust for those.

Unfortunately, you'll also lose the subtle tonal gradations in all the
middle zones, because the detail needed for that has been spread over a
larger range to cover highlights and shadows.

Whether you use negative film or slide film, you _cannot_ have it both ways.
And incidentally, digital photography doesn't change this, either.

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