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05-05-2011, 09:23 AM   #1
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Is This an Example of a Decentered Lens?

Based on the generally favorable review of the older non-os, non-HSM version of the Sigma 18-125 lens on Photozone, I went and bought this lens off ebay as a kit replacement for my K100D which I meant to to pass along to a budding newbie photographer. And by all appearances the lens received was clean, in very fine shape, and seemed well built. However optically, my copy had one bad problem. The wider the angle, the more open the aperture, the left side of the image was consistently soft and/or out of focus. In fact viewing an image from left to right, the image got progressively sharper, from poor to very good. Not a problem I could adjust for even with my K5. I take this to be a QC issue rather than a defect in design. The attached photo is just an example, take a right angle to the book case. The problem becomes less pronounced with the lens stopped down and at longer focal lengths but I consider this pretty worthless and it's hard to justify the expense of getting it repaired unless someone knows a good source.



05-05-2011, 10:26 AM   #2
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Wow, that's about the most egregious example of decentering I have ever seen. It makes me wonder if the groups are even screwed together correctly.
05-05-2011, 10:46 AM   #3
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That's optical misalignment beyond any doubt. Probably cannot be healed either. Get a refund asap if still possible.
05-05-2011, 01:00 PM   #4
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lens with a fish-eye effect sorta.

05-05-2011, 10:36 PM   #5
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Yes, my commiserations - hope you can get a refund. Modern mass-market plasticky zooms (as opposed to the conservative battleship-like vintage metal zooms) seem to suffer badly from potential decentring. If you get a good one, fine, but it's all too easy to buy a dud.

I think when it comes to buying used, there'll be a high probability that decentring will be the reason it's being sold. So the lesson for us here is that you need to know who you're buying from when you're after a used modern zoom.
05-05-2011, 10:38 PM   #6
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PS, I have exactly the same problem with a mint-condition FA28-80 which I bought cheaply a while back, so you're not alone!

Edit: actually it's an FA28-90. That reflects how often it gets looks at, let alone used!

Last edited by m42man; 05-05-2011 at 11:28 PM.
05-06-2011, 12:09 AM   #7
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QuoteOriginally posted by m42man Quote
Yes, my commiserations - hope you can get a refund. Modern mass-market plasticky zooms (as opposed to the conservative battleship-like vintage metal zooms) seem to suffer badly from potential decentring. If you get a good one, fine, but it's all too easy to buy a dud..
Very true, as my experiences with new Sigma 17-70 (two mega-duds before a good one) and new Pentax 55-300 (two slightly-duds) will testify ...

A strong argument for prime lenses if you're on a budget.

Tim
05-06-2011, 01:13 AM   #8
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yep, some alignment problem. Never seen anything like that in my experience.

05-06-2011, 02:54 AM   #9
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A good example of Sigma's QC issues...
Anyway I've never considered the Sigma 18-125mm to be particularly good.
05-06-2011, 04:32 AM   #10
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QuoteOriginally posted by creampuff Quote
A good example of Sigma's QC issues...
Anyway I've never considered the Sigma 18-125mm to be particularly good.
Whatever... I own more more Sigma lenses than Pentax lenses and the Sigma lenses have all been flawless, back up with a much longer warranty. Some of them up to 10 years.

Now about Sigma HSM vs Pentax SDM... You were saying something about QC? Pentax? Are you Serious? SDM You're one funny creampuff!
05-06-2011, 06:36 AM   #11
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I had a problem like this with an A 28 f/2.8. It was (believe it or not) even worse. Fortunately, it turned out that the front element is mounted in a ring that screws in, and it had come loose. I hope it turns out to be something easily fixable for you too.

Does it make a clinking or rattling sound if you shake it lightly (key word: lightly)? Maybe something is just loose. If you can't find anything, and you can't get a refund, you might take it to a camera shop for a professional evaluation.

Fingers crossed for you...
05-06-2011, 08:00 AM   #12
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Well unfortunately a refund is out and I'm not about to dump it back on ebay so I'll eat the loss.

I own several Sigma lenses and honestly this is the first one I've had a problem with. Until I put it on the camera and took a picture, this sample seemed really solid, nothing obviously loose, misaligned or making noise.

Guess now I have learning opportunity in taking it apart. I don't imagine it's worth the money to get it fixed. If anyone knows anyone who needs this lens for parts have them make me an offer. No good ones refused.
05-06-2011, 08:54 AM   #13
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Definitely. I had the same issue w/ a Sigma 17-70 (my 3rd copy seems ok).
What was annoying was it didn't show up all time...just specific combos of focal length and distance to align the bad parts up right so I couldn't find it unless I shot a lot on a vacation trip :-P
05-06-2011, 04:16 PM   #14
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QuoteOriginally posted by creampuff Quote
A good example of Sigma's QC issues...
I am afraid Pentax zooms don't score high on QC either. Even some DA primes suffer from misalignment.
05-06-2011, 10:05 PM   #15
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That is a textbook example of decentering, shame you couldn't get a refund. I had a DA16-45mm f/4 which had a similar problem, but I was able to exchange it for another 16-45mm f/4 which was vastly superior.
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