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05-11-2012, 04:46 AM   #1
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Possible 15mm Limited Auto Focus Issue

Hi:

I just bought a 15mm Limited to use with my K5. There often seems to be a delay in engaging the autofocus after depressing the shutter. For example, with any other lens after I depress the shutter butten halfway the lens will begin to autofocus instantly.

With the 15mm Limited there is either a slight delay or I need to disengage the shutter button and depress it again to get it to begin focusing. Is this normal? All other lenses work fine (31 Limited, DA*55, DA*50-135, 77 Limited, and DA*300).

Though I have not taken more than several dozen pictures with the lens (will take more this weekend) the autofocus does seem to work fine when it engages.

As an aside, the lens is quite soft at f4, even in the center. Not horrible/defective, but a bit dissapointing. I know it is a wide angle and f4 likely will not be used as often as the narrower apertures, but anyway.....

By f8 it is sharp acorss the frame, not bitingly so, but again, I need to shoot more.

I would note there is little to know obvious barrel distortion and CA is completely absent. I even shot some branches on tall trees directly against the bright sky and no CA was visible upon quick, but close inspection.

Thanks in advance for your help on the autofocus issue.

05-11-2012, 05:40 AM   #2
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QuoteOriginally posted by lawjbm Quote
By f8 it is sharp acorss the frame, not bitingly so, but again, I need to shoot more.
The DA 15 is one of the sharpest lenses I own, as sharp as the DFA 100 WR and a K 50/1.2 (two of Pentax's sharpest lenses). Unless you're not nailing the focus, your 15 should be, if not "bitingly" sharp, at least very sharp stopped down. It should also be reasonable sharp at f4, particularly in the center. If not, you may have a bad copy of the lens.
05-11-2012, 05:43 AM   #3
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I have found my DA15 is a bit funny about how the contacts line up when you put it on the camera. I usually have to screw the lens on, and then turn it back the other way just a tiny bit until I hear it "CLICK!" into place. That may be related to your odd auto focus issue.

My copy of the lens is crazy sharp in the center and noticeably soft in the corners. Nevertheless, I love the lens! It's color and contrast is hard to beat, as well as the "polarizing" effect it has on skies and those cool sunbursts.
05-11-2012, 07:51 AM   #4
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I just received my 15mm limited and have not noticed the AF issue you are describing. As for sharpness, it seems pretty sharp, even wide open, although, I have to admit I haven't had time to check out the edges closely yet.

05-11-2012, 03:44 PM   #5
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Hi:

Thanks all.

I will take some more pictures this weekend and see what happens and let you know.
05-11-2012, 06:39 PM   #6
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Sample variation. There are plenty of good copies out there but there are some poor performing ones too.
05-13-2012, 09:20 AM   #7
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Hi:

I took some more pictures with the 15mm Limited. It is going back.

I had previously tried the Tamron 10-24 and was disappointed with it. Even at the most frequently used narrower apertures it was too soft.

I now have the Sigma 10-20 f3.5 and, quite frankly am pleasantly surpised. I have not had much luck with Sigmas. I may have gotten a poor copy of the Pentax, but in all candor, I am tired of sample variation--among all manufacturers. Two k5s (first one did not focus with my lenses no matter what micro-adjustments, second one had the stains visible down to f8, the 3rd one has been great, though). First DA*16-50 had 2 SDM failures (optically very good, however), and a Sigma 120-400 was disappointing.

Ok, I am venting a bit, but I do like the Sigma 10-20 f3. I will keep trying it out, but it looks like a nice keeper.
05-13-2012, 02:02 PM   #8
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It would be interesting to see some of the sample shots that led you to conclude there was a problem with the lens.

05-13-2012, 04:46 PM   #9
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Hi:

Thank you Mark for the inquiry.

I have attahced a full image and three crops, f 4.5, f5.6 (file name is "5"), and f8, all at Iso 400, 1/125. I took multiple exposures of this scene and others around our front yard. I only decided to do such informal testing because when I started to take a few fun shots inside everything seemed very soft. I really have tried over the past year or so not to be a pixel peeper and get too worked up about testing lenses. My DA*50-135 is a good solid lens, not the sharperst, as it seems others might have, but its light, focuses well, is weather resistant, and gives very good overall performance.

Anyway, let me know if you think I am off. Focus was on the mulch bags and I am about 12-15 feet from them. All handheld, but all the various images looked the same and I took multiple images of each scene--they all looked the same.

I would note again, there was no visible CA and distortion (given its wide angle nature) seemed well controlled.

Also, all opened in CS5, ACR defaults (I did not correct WB). The mulch bags represent a 100% crop.

f8 is not bad, but wider apertures are not good. I did not post f4, but it was softer than 4.5 (not too much, though).

Last edited by candgpics; 09-28-2012 at 07:11 PM.
05-13-2012, 08:14 PM   #10
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I hope to have a chance to take some pictures with the Sigma 10-20 3.5 this coming weekend and will post a few when I do. Informally, it seems significantly better than the 15mm Limited. It also is not that big or heavy, though it clearly is bigger than the 15mm Lim.
05-14-2012, 04:46 PM   #11
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Hi:

It woudl be very helpful to me if others could comment on these images and (perhaps) confirm my conclusion about the lens.

Thanks so much.
05-14-2012, 04:54 PM   #12
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Seems your f/4.5 crop is sharper than your f/5.6 crop which points to a possible focus or camera shake problem in taking these. However both are not as sharp as I'd expect at those apertures. Something is clearly wrong, not sure what though.
05-14-2012, 05:07 PM   #13
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I, too, saw the 4.5 as being sharper.

Here are the 15 Limited and the Sigma at the same exposures (f4.5 and 1/125; the 15 Lim is at ISO 400 and the Sigma is at ISO 100--that was my fault), focused on the center where the two drawer untits meet. Both 100% crops. Can you pick the 15 Lim?

Thanks so much.

Last edited by candgpics; 09-28-2012 at 07:11 PM.
05-14-2012, 05:14 PM   #14
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Hi:

Here are two more at f8. Same exposures. 15 Lim and Sigma 10-20 f 3.5 at 15mm.

Which is which?

Last edited by candgpics; 09-28-2012 at 07:11 PM.
05-14-2012, 05:23 PM   #15
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The f/8 results are close, but the f/4.5 ones you had up there initially weren't with the DA15 pretty mushy.
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