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06-01-2011, 11:32 AM   #1
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Testing centering of lens, need some advice

I've just started testing my lens for misalignment. I have not found clear instructions so far, but I figured this would be a good procedure:

I have printed out a sharness test chart a few times and taped them to a wall. Then I place the camera in front of it, making sure that the camera's center is along the vertical axle coming out from the center of the chart. That makes sure that the camera sees perfectly flat surface to focus on. I take pictures from this with my lens, at a higher aperture, usually f/8 to f/11.

With my fixed-focus manual 50mm f/1.4 I get a pretty even and overall sharp picture, up to the corners.

Next, with my 18-135mm, I get a fairly even distributed sharpness as well: Perfect in the center, a bit worse at all corners. Also shows quite a high distortion ("pillow" effect) at 50mm.

Then my DA* 16-50mm, my most expensive lens. And my cheap all-rounder, a Tamron 18-200mm. Both turn out pretty badly: One or two corner are significantly out of focus.

See for yourself (each is a ~7MB file):
DA*, normal gravity: http://files.tempel.org/tmp/K5__5813.JPG
DA*, tube lifted up by hand: http://files.tempel.org/tmp/K5__5814%20%28angehoben%29.JPG
Tamron: http://files.tempel.org/tmp/K5__5819.JPG

The DA* had always worried me, as its tube, when extended, wiggles. Now I tend to believe that this does indeed lead to bad pictures. The Tamron doesn't wiggle at all, yet it has even worse out-of-focus corners.

Do others agree that these lens are off-centered?

What's the remedy? Sadly, both lenses are just over their 2-year warranty in Germany. What's your experience of the costs involved for getting these adjusted?

I have some expierence with fine mechanics, I've taking apart and repaired camera bodies in the old days when there were no electronics involved, I've also repaired laptops etc. often. I have all the necessary standard tools for that. Is there a change I could at least fix the wiggly tube of the DA* without too much trouble?

Update:
I also tested my DA 18-55 AL II, DA 18-55 AL WR, Sigma 18-200 and DA 55-300. The AL II and the Sigma were very sharp in all corners, while the 55-300 was off at the bottom corners, and the AL WR was horrible in all. Since the AL II and AL WR are supposedly the same lens, my particular WR model appear is severly broken.

Seeing that some lens do turn out very good while others quite bad suggests to me that my measuring technique was basically correct. Seeing that about half of my lens are off-center is quite worrysome to me, though. Is this off-center effect really that common?


Last edited by tempelorg; 06-01-2011 at 11:54 AM.
06-01-2011, 12:41 PM   #2
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RonHendriks1966's Avatar

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Wow what a job to do. No answers from me, but I will follow it. The corners, left top of DA* is definitly off to my idea.
06-01-2011, 02:28 PM   #3
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I think misalignment is rather common these days due to high manufacturing volume which leads to looser QC. I know the Tamron can be adjusted via 3 screws inside and some of the Tamron service centers are equipped for this job. The Pentax lenses however, are doomed IMHO. Even within warranty, they would either brush you off or exchange the lens. Re-adjust the optical axis isn't really an option for Pentax lenses AFAIK. Impact during transport might also knock them off as AF zooms are rather fragile in construction.
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