Don't you love it when we're all right? The vignetting that occurs as a
result of lens design usually appears wide open, and decreases as the lens
is stopped down. for example, the 45 with my Xpan (which has a 49mm filter
thread, compare that with the 43 for the Mamiya 7) covers nearly the width
of a 6x7 frame. Wide open there is a bit less that a stop difference center
to edge. There is center filter (very affordable at $240) that corrects for
this. Stopped down from f4 to f8 or so, the coverage is, at worst, barely
noticable. This sort of vignetting is common in wide angles. It is very
expensive to get fast, wide lenses to cover evenly wide open.
The other kind occurs when a particularly thick filter, more than one
filter, or a too-small hood is used. As the lens is stopped down, the DOF
increases, and the front edge of the offending attachment is more and more
in focus.
The simplest thing to do is take anything threaded into the lens off, and
shoot some chrome. If there is still vignetting, then think zebras.
Bill Pearce
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