Julian Loke wrote:
> But what if the dark edge is just out-of-frame? Stopping down also
> makes the edge sharper,
Or it would if the edge were in the frame ... I wouldn't expect any
vignetting with the edge outside the frame.
> Where is the EOS relevance? When shifting a TSE lens the edge of the
> image circle can come close to or enter the edge of the frame.
Depending on what the image circle actually is. The Canon specs aren't too
helpful in this regard, because they don't give an aperture at which the
58.6 mm image circle applies (I assume when the lens is wide open). A call
to tech support several years ago didn't get much of a clarification.
The Canon manual doesn't help much in the interpretation of the red marks
on the tilt and shift scales:
"Unbalanced peripheral light amount may become noticeable if the lens
is shifted a large amount. We recommend setting a small aperture value
for better picture quality." [How large an amount? How small an
aperture?]
"... shifting more than 8 mm may cause shading on the long edges or in
corners of the picture."
I'm sure the image circle for the 24 is larger than 58.6 mm at f/8, but how
much so I can't say. I don't think I see vignetting with the lens shifted
the full 11 mm in the long direction, but it's often difficult for me to
tell the difference between the edge of an image circle (it's usually not
quite as abrupt as the edge of a hood or filter) and normal cos^4 falloff.
Vignetting from an ordinary polarizing filter at full shift is unmistakable
(speaking, sadly, from experience). WJM and I were two of the participants
in a discussion of this on the list several years ago. As I recall, we had
similar impressions.
> If the edge is outside the frame, stop down to reduce vignetting. But if
> the edge is inside the frame, open up to reduce vignetting.
My thoughts:
o If the edge is outside the frame, there shouldn't be any vignetting.
o If the edge is from a filter, remove the filter (or get a thin filter
that doesn't vignette).
o If the edge is from the lens barrel, close down to f/8 or f/11, and if
that doesn't eliminate the vignetting, you're out of luck; either
reduce the shift or abandon the shot.
Jeff Conrad
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