"This is probably a dumb question, but when people say "negative film has
more latitude than slide film", which stage(s) are they referring to where
you gain/lose latitude?"
This is a Photography 101 question. It is self-answering if you understand
how negative and reversal materials work. I'm posting a third explanation
because I felt the first two were needlessly complex.
There are two basic photographic processes: negative/positive and reversal.
Negative/positive processes produce prints, while reversal processes produce
transparencies. *
Negative/positive processes require two exposures. Errors in the in-camera
exposure (over- or under-exposure, wrong color temperature, etc), can
usually be corrected when the print is made (the second exposure).
Reversal processes are single-exposure. The unexposed silver halide of the
original negative directly or indirectly generates the final image. There is
no second step during which errors can be corrected. ** Hence, there is less
room for exposure error.
You can also look at latitude in terms of highlight and shadow detail. In
order to get the biggest ISO number to print on the box, the process of
rating film speed is based on the _minimum_ exposure needed to produce an
acceptable print or slide. As a result, neither print nor slide film have
much latitude for underexposure (if you want to retain shadow detail).
Overexposure is a different matter. Both slide and print films allow
substantial overexposure before highlight detail is lost (ie, the highlights
become "blocked-up" because all the silver has been exposed and there is no
room for further incremental exposure). But this extra latitude isn't
"available" to slide films because their development is fixed -- it can't be
held back to compensate for overexposure.
For these reasons, if you aren't sure what the "correct" exposure should be,
it is generally considered safer to overexpose print film, and underexpose
slide film.
There is also a "psychophysical" aspect to exposure latitude in slide films.
Most can tolerate a two-stop overexposure without blocking the highlights.
But the resulting image -- even though it's a technically accurate transfer
of the lens's image to the film -- will usually be too light to be
acceptable, whereas a negative that has been similarly overexposed can
almost always produce an acceptable print.
* Polaroid prints fall into the reversal category. In B&W materials, the
reversal is produced by transporting the unexposed silver to the receptor
sheet. In color materials, the exposed silver traps the dyes that would
otherwise move to the receptor sheet, leaving the untrapped dyes to form the
image.
** In theory, over- or under-exposure could be partly compensated for by
seeing how rapidly the image develops and adjusting the development time
accordingly. Kodak used this principle in the 1920s when they introduced
home movies. The problem is that it requires human inspection, and only
works with orthochromatic film, which can be visually inspected under red
light. Even if you could get it to work with panchromatic or color film,
you'd still have the problem of varying exposure errors on the same roll of
film.
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