"Can someone explain to me the means of calculating the proper flash
settings (f/stop, distance of camera &/or flash to subject) when the
focal length is varied?"
Believe it or not, the original 100 (or so) questions for the "50
Question" quiz in the October "Popular Photography" had a version of
this question.
Answer: Focal length has nothing to do with exposure -- only the output
of the flash and the distance from the subject. If you don't change the
distance, you don't change the exposure -- regardless of focal length.
"Also, is the distance suggested on the flash unit the maximum distance
only, so that you could get as close as you wish and the flash's sensor
will tell the flash to cut-off at the proper amount? Or is it the
suggested distance as opposed to a mere limit on the amount of distance
to subject?"
The maximum distance is distance at which you can still get "full"
exposure at the lens's maximum aperture. In practice, the distance is
usually increased by 40%, assuming that a one-stop underexposure for
negative materials is okay.
Although the thyristor can cut the flash off very quickly, there _is_ a
point at which the subject will be overexposed. If you use slow film and
a small aperture, it's unlikely you'll ever hit this point in close-up
work. The simplest solution -- if the flash is not attached to the
camera -- is simply to move the flash back from the subject. This will
also give more-even illumination.
You can also run into reciprocity problems at very short exposure times.
(The film's data sheet will indicate whether this occurs.)
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