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02-13-2009, 03:19 PM   #1
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Need help with AF adjust on K10D

I just got the sigma 10-20, and noticed it had some pretty strong backfocus issues, so I looked around and found the program to unlock the debug mode with firmware v1.3 on the K10. Printed out a focus test sheet and went at it.

The problem arises when I change the focal length. If I adjust the focus so that it's perfect at 10mm, then it's backfocusing at 20mm. If I adjust so it's perfect at 20mm, it's frontfocusing at 10mm.

Should I just go in between and live with it, or is there a real problem here? Maybe a problem with my methods? I followed the instructions given at focustestchart.com, but they don't say what focal length to use for a zoom lens.

02-13-2009, 03:38 PM   #2
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Focus at 20mm, then back off the zoom to whatever you need. You will now have proper focus.
02-13-2009, 03:48 PM   #3
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QuoteOriginally posted by Jodokast96 Quote
Focus at 20mm, then back off the zoom to whatever you need. You will now have proper focus.
Do-able, but that would get quite tiring after a while.

Should I be at all concerned that the lens is bad? It just came in today so I have 2 weeks to return it (bought it used from KEH)

EDIT: I have the katz eye screen installed, and I can actually see in the viewfinder that the focus is wrong when using autofocus. If I switch to manual focus and correct it, the shot comes out with correct focus. That would make me think it's the body and not the lens.
02-13-2009, 03:51 PM   #4
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Is it defective? Don't know. But as I understand it, that is a long used method for focusing zoom lenses.

02-13-2009, 04:41 PM   #5
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don't bother focusing at 10mm. Just set it as follows:
10mm, f5.6, MF @1m mark.
set like this, everything from aprox 55cm to infinity will be in focus...
BR

PS: I know that's not going sort your problem, but that's how I work with my Sigma 10-20...
02-13-2009, 07:21 PM   #6
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Just a note of thanks for the advice on focusing a zoom lens. A Sigma 18-50 f2.8 has been driving me nuts. Your method of zoom full out an then back off worked.
04-01-2009, 01:50 PM   #7
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I have the same issue, and while all those solutions will help take better focused pictures, it doesn't change the fact that there is a problem. My camera focuses fine at longer focal lengths, and is horibly back focused at wide FLs. Should I just live with it? Like the OP said, if you correct it at wide angle, then the focus will be off at the other end. If it were just a problem across the whole range, then it would be easy to fix the whole thing, but since it's on one sie and not the other, what do you do?
04-01-2009, 07:54 PM   #8
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I've recently found out that a lens that goes from front to back focus isn't exactly uncommon. I would set the adjustment so its around the centre. Of course with a lens like the 10-20 you probably never use it at 20 anyway, and with its dof you'll probably never notice in the real world. Another factor to consider when setting the af would be what focal ranges you actually use with that lens, most people never take a photo with it at 20mm

I wouldn't worry about it too much anyway.

04-01-2009, 08:31 PM   #9
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I agree that focus accuracy on the 10-20 isn't that big of a deal. However, front/back focus issues are pretty common problems with Sigma lenses. If it were me I'd return it and try for a better copy before you start monkeying with your firmware. My second copy worked perfectly.
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