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03-13-2010, 04:57 PM   #1
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Sigma 17-70 BF only with distant subjects?

Last summer I bought a Sigma 17-70 and during my first outing with it, I noticed some very disappointing back focus. I did a focus test, though, and it seemed fine. After quite some time, I think I've finally decided that it only back focuses with distant subjects. At macro distances (where focus tests are performed), it seems fine. Is this considered a defect? If a lens BFs, should it at least be consistent? Can I ship the lens back to Sigma to have it adjusted? Or will they swap it with a refurb instead? I've haven't checked the warranty, but surely it's at least a year, right? And I bought it from Amazon, so I should be able to produce proof of purchase with no problems.

Thanks.


Last edited by lavascript; 03-15-2010 at 06:56 AM.
03-14-2010, 11:40 PM   #2
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QuoteOriginally posted by lavascript Quote
Last summer I bought a Sigma 17-70 and during my first outing with it, I noticed some very disappointing back focus. I did a focus test, though, and it seemed fine. After quite some time, I think I've finally decided that it only back focuses towards the long end. Is this considered a defect? Can I ship the lens back to Sigma to have it adjusted? Or will they swap it with a refurb instead? I've haven't checked the warranty, but surely it's at least a year, right? And I bought it from Amazon, so I should be able to produce proof of purchase with no problems.

Thanks.

The warranty is one year on non EX Sigma lens. So you have a short time. I would send it in to be checked no matter what. They may swap it, But if you send it to C.R.I.S in Arizona (Sigma service center I highly recommend) the chance is less than if you send it to NY; it comes back cleaned, the zoom tighten if there was any zoom creep and of course your complaint addressed, I have found it better to be fixed than swapped IMHO. Not much to loss other than two weeks if no parts from Japan are needed or 4 weeks if parts are needed not stocked.


Now the Sigma is usually a bit weak at 70mm f4.5, its OK in the middle but soft going toward the corners. At f5.6 my copy just snaps into crisp focus corner to corner (well good) and very good at F8.

If they tell you your dSLRs AF needs adjusting believe them, as I did not and found out long after they were right.
03-15-2010, 06:54 AM   #3
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QuoteOriginally posted by jamesm007 Quote
If they tell you your dSLRs AF needs adjusting believe them, as I did not and found out long after they were right.
Can I adjust it myself (K10D debug mode) or do I really need to send them together?

And I'm an idiot. I totally didn't mean tele in my post title, I meant long subject distance. It focuses spot-on at macro distances at 70mm, but definite backfocus with far-away subjects, but a building or wall or tree behind the subject is sharp.
03-15-2010, 11:14 AM   #4
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QuoteOriginally posted by lavascript Quote
Can I adjust it myself (K10D debug mode) or do I really need to send them together?

And I'm an idiot. I totally didn't mean tele in my post title, I meant long subject distance. It focuses spot-on at macro distances at 70mm, but definite backfocus with far-away subjects, but a building or wall or tree behind the subject is sharp.
Don't send them together as Sigma will only take a picture with your K10D and not touch anything else. Thats when they advised me to have my GX10 AF serviced. One side was soft, and only with that one lens out of 5 so I argued how can that be! Ended up that if you have slight AF issues BF/FF it can do that as the frame is not in perfect focus. I asked for proof and they sent the below, which did not prove anything at the time for me. On their K10D the left side was not as soft as on my GX10, and the whole pic with the K10D was a tiny bit sharper, a very small amount...

Yes you can use the debug menu to correct it as I did. I still have to dial in about +4 on my K20D if I want it perfect wide open. If your problem goes away at around f5.6 and higher where the DOF should eliminate BF and if you can't send it in, go ahead and use the Debug Menu. Although you will have to change the setting everytime you put on a different lens. But I do to with my K20D. The K20D can remember 20 lens, but can only differentiate Pentax lens. I have two Sigma lens and everytime I put them on I have to go through the menu and make sure the AF fine tunning is set right .

Pic from my GX10 17mm f2.8

Pic from their K10D 17mm f2.8


Just showing what they will do. Now Pentax will calibrate your K10D to the Sigma lens. But it will cost...

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