On 8/14/2010 7:43 PM, Michael Wong wrote:
> I think you point to picture "XL72F56_065". The picture was shot by Super
> Angulon 72mm XL with UV filter& minor shifting. I noticed the vignetting but
> I'm not sure the cause from UV filter or shifting.
I know veerrry little about working with shifts and tilts. I do know some other
stuff. :-)
I don't think this is lens or filter vignetting. It manifests only on one long
size, and is linear from top to bottom,
not circular. Here's a version with brightness adjusted by a linear gradient.
<http://www.moosemystic.net/Gallery/Others/MWong/XL72F56_065.htm>
I'm not trying to make a "better" image, only to show how even the effect is
from side to side and how linear from top
to bottom. Even if lens vignetting were asymmetric to the image due to
shifting, it would show a circular pattern,
darker near the corners. Also, look at the top part of the central building.
It's distinctly darker than the bottom in
the original.
The thing that comes to mind is the effect of perspective correction. The
verticals are quite, well, vertical. So some
adjustment of lens relationship to film plane has been made to correct for
perspective distortion. That means the top of
the image has been expanded relative to the bottom on the film (or the
reverse). That means less brightness the farther
one goes from one end to the other along the axis of correction.
Whether the inverse square law here is enough to cause such a great difference
in exposure, I have no idea. Certainly
I've never read of such an effect, but I don't read about LF, shifts and tilts.
Speculative Detection Moose
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