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06-03-2009, 09:42 PM   #1
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Are one or more of my lenses defective? Is it the camera? Or is it me?

All,

I'm trying to diagnose my dissatisfaction with my 18-250, which has been repaired once and replaced once by Pentax and still seems to be off-center. I ran some tests with 4 of my lenses (DA 18-55, Sigma 18-125, DA 18-250, Sigma 17-70) to try to get to the bottom of things - one with a blue-sky landscape scene (a common composition of mine) and the other of an evenly illuminated white door, with PP to emphasize the vignette (at a variety of focal lengths and f-stops).

The former test seems to consistently show more vignetting on the right side of the frame, across all lenses, while I'm not sure what to make of the latter. The original 18-250 also was darker on the right side.

I'd like to blame something in the test setup, but I'm not sure what it could be.

Anyway, I would be extremely helpful for your help in figuring out what's going on. The test images, summary, etc. are at: Lens Vignetting Tests (note: it's a very large web page with a couple of hundred 400x300 JPEGs).

Feedback on the test setup or advice on what might be wrong is quite welcome. It makes me sad and frustrated that I've spent so much money on a nice DSLR and now I have to deal with a problem that I never once noticed on my $150 P&S.

Thanks,

Reid

06-03-2009, 09:58 PM   #2
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I've noticed that the vignetting is inconsistent among my lenses as well. I've always wondered whether the SR system de-centers the image or not.
06-03-2009, 10:27 PM   #3
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It could be SR, since the sensor shifts and may see different parts of the image circle. Try your tests with SR off.
06-03-2009, 11:12 PM   #4
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Same problem about Pentax 18-250 here. One or more reviewers on pentaxforums and amazon also point it out.

06-04-2009, 08:33 AM   #5
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Original Poster
QuoteOriginally posted by krypticide Quote
It could be SR, since the sensor shifts and may see different parts of the image circle. Try your tests with SR off.
SR was off on almost all the images; just a few on the Sigma 18-125 before I remembered to turn it off IIRC. No perceptible difference.

QuoteOriginally posted by boom Quote
I've noticed that the vignetting is inconsistent among my lenses as well. I've always wondered whether the SR system de-centers the image or not.
It's not that the vignetting is inconsistent -- that's what I would expect given random variations between lenses -- but that it's consistently the right edge of the frame that's darker across four or five lenses. But, then when I specifically tried to check for symmetry it's not so consistent. So I'm confused, because I'm not sure what would have caused the asymmetry in the blue sky test.

FWIW, if I set a long manual exposure and trigger the shutter with SR on and with no lens attached, there's no visible movement of the sensor, but I do hear a faint buzz which I assume is the SR operating.
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