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Re: [OM] Visited Macau

Subject: Re: [OM] Visited Macau
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Sun, 15 Aug 2010 07:54:02 -0400
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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