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07-16-2011, 09:19 PM   #1
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Sigma 85mm front focus on K-5 update

I believe i've posted about the Sigma 85mm 1.4 front focusing like heck on the K-5 body before. This seems to happen only when I'm shooting faces (at ANY distance), even closeup. When I do a test with flat piece of paper, a wall with plenty of contrast or anything similar, the Sigma nails it. There still seems to be few people using the sigma 85 on the k-5, but I was wondering if anyone else has noticed this. Initially I thought the problem was the K-5's center point focus area being too big and it's inability to focus on the eye itself which is what you MUST focus on for accurate focus at wide apertures. I've tried and tried and the sigma 85 just front focuses like heck, with the exception of 2 or 3 portraits out of 30 or 40 shots, all of them with the head filling about 50% of the picture. Most of the shots stop about an inch before the eye, leaving the entire face out of focus with the exception of a few wandering hairs. I've used the Tamron 70-200mm 2.8 extensively and focusing is fantastic with that setup, so I'm assuming it's the Sigma lens. It seems to get confused in a relatively simple scenario. This is a shame because I'd really like to use this setup. I've tried focus adjustment and the lens focused the same at all adjustments (had no effect on performance).

07-16-2011, 10:05 PM   #2
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QuoteOriginally posted by outsider Quote
the Sigma 85mm 1.4 front focusing like heck on the K-5 body
On your K-5 body. FF/BF etc issues are all about the interaction between individual samples of lens and body, and the varying manufacturing tolerances that may apply to both items.

Anyway, it also sounds like you are confusing a DOF issue with a AF issue. If the Sigma nails the flat sheet of paper but fails to nail the [lumpy] face, then it sounds like you haven't factored in the razor thin DOF you are likely to get at f1.4 at 85mm. Stop down a notch or two to boost your DOF and the issue will likely go away.
07-16-2011, 10:58 PM   #3
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QuoteOriginally posted by rawr Quote
On your K-5 body. FF/BF etc issues are all about the interaction between individual samples of lens and body, and the varying manufacturing tolerances that may apply to both items.

Anyway, it also sounds like you are confusing a DOF issue with a AF issue. If the Sigma nails the flat sheet of paper but fails to nail the [lumpy] face, then it sounds like you haven't factored in the razor thin DOF you are likely to get at f1.4 at 85mm. Stop down a notch or two to boost your DOF and the issue will likely go away.
I want to make very CLEAR, that this is not a depth of field issue. I had a feeling someone was going to bring that up, as they usually do when discussing this topic (and rightly so), but this IS a front focus issue. Nearly 90% of all my head portraits come out so badly front focused with the sigma/k-5 combo, they're all garbage. There is NO reason I shouldn't be able to nail half of all of such shots working at 1.4 aperture. I can nail the eyes from 7-10 feet back at 1.4 with this sigma on a k-x body (assuming I can keep the focus point steady enough, which is not that hard for me) and sadly, that's the combo I'm forced to use most of the time. There shouldn't be any reason to think that this is some kind of special accomplishment anyways for two modern pieces of equipment. I do portrait photography by the dozens of faces a month and nailing focus at wide apertures is not difficult enough to have to stop down. Not even close. There's no reason to even consider stopping down when this lens back focuses this badly dude. It's not depth of field. Stopping down would be a roundabout "fix". No offense, but it's kind of a ridiculous "solution" in this problem. Things would be less blurry because of aperture but still not how they should be.
07-16-2011, 11:59 PM   #4
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For what it's worth, I've got the Sigma 85/1.4 and don't have the issues that you have on either of my two K-5 bodies, or my K-7 body. When I first got the the lens though, I did have have issues with getting what I want in focus due to the very narrow DOF, but and that was definitely a technique/user issue.

07-17-2011, 12:19 AM   #5
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Maybe here's another reason to stop down the 85/1.4 for portraits, unless you explicitly want a soft-focus effect, alongside your FF:



Also remember that the AF focus points on the K-5 and K-x aren't actually laser-like points. They are regions of contrast for the AF algorithms to work on..
07-17-2011, 12:30 AM   #6
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QuoteOriginally posted by rawr Quote
Maybe here's another reason to stop down the 85/1.4 for portraits, unless you explicitly want a soft-focus effect, alongside your FF:



Also remember that the AF focus points on the K-5 and K-x aren't actually laser-like points. They are regions of contrast for the AF algorithms to work on..
Thanks for the info, much appreciated!!!
07-17-2011, 12:32 AM   #7
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QuoteOriginally posted by pop4 Quote
For what it's worth, I've got the Sigma 85/1.4 and don't have the issues that you have on either of my two K-5 bodies, or my K-7 body. When I first got the the lens though, I did have have issues with getting what I want in focus due to the very narrow DOF, but and that was definitely a technique/user issue.
Interesting. Thanks. I've heard that no two copies of a lens works quite the same, but it's strange that my sigma 85 works fine on the k-x. I'll have to keep trying (or praying). Possibly get another copy...
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