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05-20-2014, 09:58 AM   #1
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Sigma 10-20mm f3.5 back focus issue

Hello,
So recently I got a Sigma 10-20mm f3.5 EX HSM for my Pentax K-5 and soon had to notice that the AF doesn't work quite right.
It back focuses quite a bit, which makes it fairly useless most the time, unless you want blurry pictures.
So what do I do about it? Send it back and get a replacement? Can I send it to Sigma to fix it for free? Is it common for this lens? May it be that the AF System of the K-5 can't handle UWA?

Thanks in advance!

P.S.:
A Picture with a grid paper:


the center AF point was selected and it's supposed to be focused on the center cross.

05-20-2014, 10:27 AM   #2
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Have you tried AF adjustment For back focus you need to +
menu C4 AF fine adjustment Go to +3 (example) and take test shots etc etc again till in focus
05-20-2014, 10:31 AM   #3
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Well yeah, looks like +6 does the trick, but is that really normal?
05-20-2014, 11:58 AM   #4
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Yeah, there is variation among lenses and bodies. I have several that have a +6 adjustment on my k's. If that fixed the problem your good to go.

David

05-20-2014, 12:10 PM   #5
MJL
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I used this lens/camera combination before. I have to use manual focus for any image which include a lot of small objects such as your grid paper, or object with low contrast. My observation is the AF system in K-5 needs fairly good contrast and good size object to nail the focusing point. Therfore, it is no surprise that your K-5 picks the wrong focusing point with a UWA zoom like this Sigma.

I am currently using another combination of UWA zoom/camera and still encounter focusing issue with small objects, although it is a lot more less.
05-20-2014, 12:27 PM   #6
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QuoteOriginally posted by MJL Quote
My observation is the AF system in K-5 needs fairly good contrast and good size object to nail the focusing point. Therfore, it is no surprise that your K-5 picks the wrong focusing point with a UWA zoom like this Sigma.
I agree- sometimes Pentax cameras struggle with ultra-wide lenses, but it depends on the lens. Fortunately, the hyperfocal distance usually saves you

Adam
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05-21-2014, 04:00 AM   #7
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read this:
LensRentals.com - How to Test a Lens

Consider the angle of your test shot, It may be taken at too flat an angle and you may be getting normal depth of field. Also you need to use a test chart more suitable for accurate testing. Read this.

http://regex.info/blog/photo-tech/focus-chart

Last edited by Oldbayrunner; 05-21-2014 at 04:20 AM.
05-21-2014, 08:59 AM   #8
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Oldbayrunner, that test shot was basically to just make it very obvious so I can show you. In reality the issue just is that everything is out of focus when using AF alone without correction. Thanks for the links though, will definitelly test my lenses like that in the future

Thanks for the info guys, I keep the lens now and just use the correction and some manual focus skills!

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