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04-30-2013, 08:13 PM   #1
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Sigma 10-20 and K5 IIs

Hi folks,

First of all, I hate testing and pixel peeping, I do it only when I notice a real issue in actual use. I do enough testing in my day job as a developer.

So, I have a K5 IIs and a Sigma 10-20. For a bit of background, I used to own a K5 and another 10-20 until last year, when I had to sell my setup. I then used an Olympus OM-D for a while. I am now back with the Pentax.

Here's the issue I'm having, and I'm wondering if anyone else here has noticed something like this: When using the lens at the wide end, it seems that the combo has a lot of trouble focusing in the distance. I first noticed it when I realized the picture sides were noticeably softer, or less in focus than the rest, often accompanied by a slightly sifter center. It doesn't happen all the time.

What I've noticed is that when auto focusing in the distance (let's say on the mountains), the lens seems to want to settle on some hyperfocal distance rather than actually focusing over there. The ring will go to 3ft/1m. When this happens, well, then that's where the best point of sharpness is, and you get more blur at infinity, and the sides suffer even more (it seems a slight misfocus of the center is way amplified when reaching the sides of the picture, and I'm not talking about the extreme corners here), beyond what's acceptable on an UWA lens.

If I switch to 20mm, there doesn't seem to be any issue there, and the scale will jump to near or at infinity, as it should. When properly focused, the lens is pretty sharp. If I then switch back to 10mm, generally the AF will not move and just stay there near infinity.

So, I'll specify that my previous 10-20 on the K5 had no such issue and worked really well (I have also compared with shots I took last year and reproduced them with same settings).

Also, this is a replacement as I sent back the first copy for this same issue and also noticeable back focus at 20mm. This second one has lens backfocus.

So, wondering if anyone has seen this issue, or perhaps an incompatibility between the K5 II's AF system and the lens, or similar behavior with an UWA zoom from another brand.

I will try to do some testing with manual focus in the morning. I'm not sure about alternatives. I'm a wide angle shooter. The Pentax 12-24 is obviously not as wide (and has a goofy hood, as an aside) and is way more expensive...


Last edited by LeChuck; 04-30-2013 at 08:25 PM.
04-30-2013, 08:19 PM   #2
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I've had this issue with the 12-24mm on my K-5. Did a minor AF adjustment and that seemed to fix it.

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04-30-2013, 08:24 PM   #3
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Adam,

The AF adjustment doesn't seem to do any good. I first used it to correct the back focus. It made the lens front focus at 10mm and I couldn't get an infinity shot in focus at all. Can't use that on zooms really.
04-30-2013, 10:38 PM   #4
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I get the same thing with my Sigma 8-16mm f/4.5-5.6 ASPH DC lens - I just consider it a consequence of pentax DSLR cameras having larger AF sensors than competing brands. 90% the AF works fine with that lens but there are several times when I need to switch to manual focusing to correct for these errors, and it nearly always involves distant subjects. Then again it is possible you have a de-centred lens which could be throwing off the AF - there is always that possibility. I went through several copies of the Sigma 8-16mm to find one that delivered performance that was up to spec. Allegedly Sigma QC tested the last one they shipped to the store I got it from to be certain it was a good copy.


Last edited by Digitalis; 04-30-2013 at 10:45 PM.
05-01-2013, 06:47 AM   #5
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I can't chalk it up to that. Something is defective here. The K5 I had with the 10-20 never had that issue, and right now it is more around 90% of failures. I didn't think the K5 II's focusing system had changed that much. In my tests, the whole central AF area has enough to focus on in the distance, and yet it persists in setting the focus distance to 1m and never more at 10mm, so instead of the house I'm focusing on, it's the grass at my feet that's sharpest and is nowhere near the focus area. Also, it doesn't seem that switching to live view with contrast AF changes anything. In the same scene, if I switch to manual focus and set it closer to infinity, then focus will actually be where I expect it.

I already sent back the first one and this is a new one I received yesterday, showing the same phenomenon. I'm trying to make sure the body is not defective at this point before I send the lens back. My fisheye lens is manual focus, and my 35mm does not have a scale, so it's hard to compare with those, and in any case, I'm not seeing this issue at 20mm.
05-01-2013, 07:03 AM   #6
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Morning,

About a year ago I picked up a Sigma 8-16, which focused similar to yours. First try manual focusing to see if you can get a sharp image. That will tell you if the lens is able to focus at all. If so, it may be similar to mine where for auto-focusing the lens was about 45+ units out of adjustment. In the camera body, you can only adjusts up to 10 units in one direction (back or forward). Since you have adjusted for backfocusing and the lens is still apparently out - you have a larger adjustment problem than what you are able to apply a fix for.

I took my camera and lens down to CRIS, they service both Pentax and Sigma under warranty. They found the lens adjustment problem. The removed the AF adjustment on the body, and then adjusted the lens to the body. Now both the lens and body work perfectly together.

Another problem with Sigma is that the K5 body recognizes up to 10 individual lens adjustments, however for Sigma lens, only one adjustment for all Sigma lenses. So adjusting the lens to the camera is the only solution if you have multiple Sigma lenses.

05-01-2013, 07:11 AM   #7
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This one is not corrected. I noticed the backfocus only on closeups at 20mm. I had corrected the backfocus on the previous one but it totally threw off distance focus afterwards. This one is left uncorrected as there is no way to correct both 20mm and 10mm independently.

I don't have time for more this morning but here's a link to a quick test shot. It's straight out of Lightroom with everything at default.

http://www.pichevinphoto.com/webupl/_IGP0765.jpg

This was taken at 10mm, f/6.3, ISO 200 (plenty of speed at 1/350). Focus is right ahead around that little pink shed. The center seems to be sharp and in focus. If you blow it up 100%, notice the tree to the left of the shed. It starts off detailed enough, but see how it very quickly goes to total blur when you move to the left, way before reaching what we'd call the corners, I'd say already at about a third from the left side. The right side doesn't show such out of focus areas until you get way into the corners. Also, looking at the ground doesn't show such issue, so this is no longer a DOF problem.

This is a bit inconsistent and manual focusing at infinity doesn't show this issue (I'll test more tonight with the exact same position).

An extra concern is that both Sigmas have shown the same behavior here.
05-01-2013, 10:52 AM   #8
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QuoteOriginally posted by LeChuck Quote
The center seems to be sharp and in focus. If you blow it up 100%, notice the tree to the left of the shed.
Yes DOF would be sufficient at 10mm f/6.3 to cover that scene adequately, looking at the sample image I would say you have a rather catastrophically de-centred lens. I suspect if you had an image of the same subject with the flipped the camera upside down you would get a better idea of how bad it is.

05-01-2013, 11:02 AM   #9
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I had that same idea of using the camera upside down to check with the exact same subject using the other side of the lens. Might do it after work, but I also received a 28mm prime with a focus scale, which will help me check that the camera focuses properly, at least check that scale distance behavior (and that the left side is not blurred as well).

That said, the decentering issue is so well known with this lens, it is most likely that. What bothers me is it's exactly the same as the previous lens but I guess with enough bad luck, it's possible. The only difference I noted was that the previous one had worse back focus on closeups at 20mm.

I wish I had my old 10-20 back! As an aside, and it may just have been good luck/bad luck, my good one had the old crinkly finish, unlike these newer ones...

The added issue, is there aren't really any alternatives, especially with the price hike on the Pentax lens.
05-01-2013, 05:24 PM   #10
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The test that you were all waiting for!

So I started off by taking the same photo as this morning, 10mm, f/6.3 auto focus on the shed, same result as this morning (I later realized I had switched the AF mode to auto rather than select, but it did select the center point each time anyway).

We can see the left side is very blurry, starting less than midway through the tree.

http://www.pichevinphoto.com/webupl/_IGP0775.jpg

Second photo. This time I used the camera upside down. Since it's a bit awkward, I took a couple shots and selected the best one, and there's perhaps still a bit of motion blur.

Even in the lens than ideal condition, with perhaps a touch of blur from movement, we can still clearly see that the tree is much better, including the whole area to the left. Now the area on the right (the old door, and the grass near it, pardon the poor state of my backyard, photography and woodworking haven't left me enough time for yard work! and it's already so darn hot...) is more blurry than on the previous photo. So, the blur remained on the same side of the lens:

http://www.pichevinphoto.com/webupl/_IGP0776.jpg

Third shot. This time with the camera right side up again, but manually focused to the infinity point, still at f/6.3. Blur is gone and things are crisp enough, in the distance and on both sides. I supposed the DOF this time is proper and must hide the de-centering, or there's something else going along those lines:

http://www.pichevinphoto.com/webupl/_IGP0778.jpg

And just for a control, last photo was taken with my new SMC F 28mm f/2.8, also at 6.3, focused in the same spot with AF, the scale on the lens moved to the infinity point. All crisp.

http://www.pichevinphoto.com/webupl/_IGP0780.jpg

I think I can rule out a camera issue with some side softness or anything like that. It does seem related to the left side, and to the Sigma lens, and more obvious when lens is not focusing to the intended focus point...

Last edited by LeChuck; 05-01-2013 at 05:32 PM.
05-01-2013, 06:58 PM   #11
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QuoteOriginally posted by LeChuck Quote
I think I can rule out a camera issue with some side softness or anything like that. It does seem related to the left side
yep, your lens is de-centred an optical fault caused by slack quality control - which is a rather common issue amongst sigma lenses.

Last edited by Digitalis; 05-02-2013 at 04:23 AM.
05-01-2013, 07:12 PM   #12
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It's in the box to go back to Amazon...and then we'll see what to do for a replacement...
05-01-2013, 08:58 PM   #13
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Is that the Sigma 10-20mm f/4-5.6, or the Sigma 10-20mm f3.5?

I got lucky with my Sigma 10-20mm f/4-5.6, it seems.
05-01-2013, 09:05 PM   #14
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It's the f/4-5.6...
05-02-2013, 08:03 PM   #15
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QuoteOriginally posted by interested_observer Quote
...
Another problem with Sigma is that the K5 body recognizes up to 10 individual lens adjustments, however for Sigma lens, only one adjustment for all Sigma lenses. So adjusting the lens to the camera is the only solution if you have multiple Sigma lenses.

Not on my K5. I have two Sigma lenses and they have different corrections applied.
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