back focusing problem?


Zeroplus1

New Member
Jan 29, 2010
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#1
Hi folks, gotten my Sigma 10-20 2 weeks back from an online seller. I think there's some back focusing problems. I have sent the following photos to the seller but he claims that there is no issue with the lens as I did not use the correct way to check focus. Can

someone give some comment/suggestion? Do I have a case here to send the lens for calibration and ask the seller to honour the warranty due to manufacturing defect?

1. focus on letter N, but area of focus is behind letter N



2. focus on 8, but area of focus is around 7



3. focus on 10, but area of focus is around 9
 

pinholecam

Moderator
Staff member
Jul 23, 2007
10,944
88
48
#3
My few cents :
1. Its harder to get exact focus point on a UWA like the Sigma 10-20. Probably due to large DOF of a UWA.
2. Sigma 10-20, quite well known for FF/BF issues. But its can be matching issue with camera too, so not all cameras will see it.
Try to calibrate it out with the camera fine adjustment.
3. Camera AF area is rather big (ie. larger than the red square indicator), so it may not be focusing where you think it is.
4. Worst case, return to seller (if he accepts) or send to Sigma for a lens calibration.
 

fengwei

Moderator
Staff member
Aug 25, 2004
14,462
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Queenstown
www.pbase.com
#4
Do NOT shoot at an angled subject for AF verification 'cause the AF area is bigger than you see (the small red/green square in the viewfinder). When do AF check, always shoot at a flat surface which is straight on (a newspaper on the wall or something similar ...). Or you can put a box (or something w/ flat surface) at an angle to a ruller, and shoot at the box, then check the ruller and see how much off is the focusing. You can read more about this here:

Luminous landscape

Or here:

http://denisglennon.com/files/Curing-Front-Focus-Back-Focus-.pdf

The shoot-a-ruller-at-45-degree method is a not the proper way to check FF/BF, and it's not suitable for most cameras (which have rather big AF areas)
 

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