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08-31-2011, 09:22 PM   #1
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Sigma 17-70 2.8-4 autofocus trouble?

I'm having autofocus trouble with my Sigma 17-70 (the newer HSM version) on my K-7. It is sometimes just refusing to focus--either works pretty quickly or not at all. Thinking it was low light, I tried focusing on a lampshade with plenty of light passing through, and had the same results. By contrast, the kit lens I was meaning to upgrade focuses very quickly. The Sigma is sharper, but I seem to have to use manual focus half the time (the TTL focus confirmation still works fine with the Sigma lens on manual focus).

Has anybody else had similar trouble? I've read some reviews that mention some searching for autofocus in low light, but they didn't seem to suggest as much trouble as I'm getting, and the light wasn't really that bad. It's obstinate enough to make me wonder if I've got a poor copy.

Josh

08-31-2011, 11:11 PM   #2
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Does this happen with other sdm/hsm lenses? It would be worthwhile to test, if possible. There might be something wrong with the lens, or with the camera.

To eliminate the trivial suspects, try cleaning your lens contacts and making sure your battery is fully charged.

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09-01-2011, 05:19 AM   #3
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The lens is new--I'm having the trouble with it out of the box. I don't have any other HSM lenses to compare, and unfortunately I don't know of a Pentax dealer in my area--I had to order the lens online. The camera doesn't have trouble with any other lenses I own. It focuses noisily but quickly with my non-HSM Sigma 10-20 even in fairly low light.

The contacts are clean, and the battery is full. The problem is most pronounced at the longer end of the lens.
09-01-2011, 06:12 AM   #4
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Sounds like your lens has a problem if everything else works well in your kit. Make a few controlled tests in good lighting conditions, and compare the performances of other lenses (accuracy, AF time, etc). then contact your reseller or directly Sigma.

09-02-2011, 12:44 AM   #5
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My copy seems to have issues focusing at the lower end of the zoom range (17mm). Even when I pick a bright clear focus spot at the camera is sayings its in focus when I view it on the computer and zoom in a litte its way off... If I manual adjust I can hit it perfect.

If i zoom in to about 50mm on the exact same object the focus is spot on. I want to desire to keep this lens on my camera but I always seem to take it off as I just can't trust it. Never knew if it was the lens as a whole or just my copy/combination with k5.
09-26-2011, 03:50 AM   #6
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I just got the Sigma 10-20 HSM version, and have the exact same problem. Sometimes it focuses really fast, and many times it just stops trying to focus at all. Very weird, my older version 10-20 focuses MUCH faster and more reliably.
09-26-2011, 10:55 AM   #7
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QuoteOriginally posted by mangeh Quote
I just got the Sigma 10-20 HSM version, and have the exact same problem. Sometimes it focuses really fast, and many times it just stops trying to focus at all. Very weird, my older version 10-20 focuses MUCH faster and more reliably.
Well, I know that's not normal. I used to have the 10-20 HSM and it always focused quickly and accurately on either K7 or K5. The first copy of the lens that I got had to be sent back for an unseemly grinding noise, but the replacement was perfect. I just sold it because I prefer the DA15 Ltd.
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