I've always believed that the focal plane shutter speed is based strictly
on the time it takes for the entire slit to pass any point on the film. If
you look at a table of shutter speed settings for, say, a Speed Graphic or
a Graflex, you will see that for any tension setting the speeds get slower
as the slit gets wider. That's because it takes longer for a wider slit to
pass any point than it takes for a narrower slit to pass that point.
In the Graflex/Graphic table, the shutter speeds for any tension are
almost, but not quite, inversely proportional to the width of the slits. I
suspect the "almost but not quite" is due to (1) the movement of the
curtain is not constant since it starts at zero and (2) some kind of
averaging is used.
Marty
