I know little about this either but I do know that, when using a 90mm
lens on a 6x17 camera (86 degrees horizontal coverage) it is customary
to use a "center filter". These are very expensive gradient ND filters
designed to compensate for the vignetting of such a wide angle lens. As
it turns out the angle of coverage of a 72mm lens on a 4x5 camera is
very close to this at 83 degrees. So I'm fully prepared to believe that
this lens causes vignetting but I was surprised to see it only in the
upper right hand corner. It seems there must be some non-symmetrical
tilting or shifting going on.
Chuck Norcutt
Moose wrote:
> 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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