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02-11-2011, 02:32 PM   #1
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10-20mm back focus, what should I do?

just got my sigma 10-20mm 4-5.6 today from B&H, did some focus test with batteries in both natural, tungsten light, find the lens back focus 8-10mm
Now, what should I do? this lens will mostly use for infinity and hyperfocal distance...should I still return it and try a new copy? or just live with that back focus?

02-11-2011, 02:37 PM   #2
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if you have no AF adjust on your camera I would just return it and try again. Especially if the focus issue happens in natural light as well as tungsten. Lots of ppl here have that lens, im sure they will chime in. What body are u using?
02-11-2011, 02:41 PM   #3
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Using K-x and i've tested all my other lens, 50mm A1.7, M28mm, kit lens all focus correctly in both natural and tungsten.
the only major issue i got returning it is that I have to wait like 1-2weeks for an exchange.(BC Canada)

Last edited by OmegaKulu; 02-11-2011 at 03:01 PM.
02-12-2011, 09:33 PM   #4
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I only have a k100 and k200, but autofocus just doesn't work for me with this lens at the bottom of the focal length range. The bodies focus very differently, but not reliably in either case. Ironically both bodies focus better with it at longer (therefore darker) focal lengths.

I'd suggest testing very, very carefully for uniformity of focus across the frame (especially in the corners) at every focal length. It probably won't be perfect, but if you have four relatively consistently sharp corners, keep the lens. You don't want to trade what may be one tiny problem for a much more significant one.

Paul

02-14-2011, 03:50 AM   #5
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sigma 10 20

I also have the sigma 10-20 and a k-x and I find that focus is problematic in certain circumstances
a) at the widest angles of view i.e. around 10-12mm focal length.
b) when the object is very close, i.e near the minimum focus distance
c) in dim or unnatural lighting.

I've found that the best way around the problem is either to use live-view contrast detect focus or zoomed-in manual focus in difficult situations or to simply stop the lens down to f11 so that the depth of focus makes everything in focus anyway.

I agree with the other poster that checking for severe asymmetry in the image sharpness is important.

Oliver
02-14-2011, 06:05 AM   #6
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QuoteOriginally posted by OmegaKulu Quote
just got my sigma 10-20mm 4-5.6 today from B&H, did some focus test with batteries in both natural, tungsten light, find the lens back focus 8-10mm
Now, what should I do? this lens will mostly use for infinity and hyperfocal distance...should I still return it and try a new copy? or just live with that back focus?
I would suggest that you post a few examples of your problem in the Sigma 10-20 lens club list in this forum and state your problem. There are very insightful members there that can give you expert opinions.
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