leaf shutters

[Stanley E Yoder <syoder+@andrew.cmu.edu>]

    There has been some comment about leaf shutters not exposing evenly,
i.e., less exposure at the edges of the frame than in the center.
    There was a very good (IMHO) discourse on this recently on one of
the rec.photo newsgroups by one David Rosen, and what I say below is a
summary paraphrase of him. Makes sense to me, but what do I know? So,
*FWIW*: 
   The accusation is not *usually* so, in that leaf shutters are
commonly located adjacent to the iris. Such BETWEEN-the-lens shutters
can be considered a second iris, albeit one that closes all the way and
has a timing mechanism governing its opening/closing. So why doesn't the
*real* iris also cause uneven exposure? Because both are located at or
near the convergent point of all light rays entering the lens. In fact
some old b-t-l shutters were also the iris, in that their *amount* of
opening was controlled by the iris setting (there being no separate
iris).
    The issue of the proportion of total exposure time consumed by the
blades in motion, opening and closing, affects the effective shutter
speed. The marked times on shutters are often a fudge of theoretical and
effective speeds.
    True, BEHIND-the-lens leaf shutters are a different matter. Uneven
exposure is  (at least theoretically, and possibly actually) possible
with this location.

Stan Yoder
Pittsburgh