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07-13-2014, 04:08 AM   #1
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Inconsistent AF on DA21mm ltd

Hey guys, recently i bought the 21 ltd used from ebay and played a little around with it and felt that (only) sometimes the focus missed eventhough the camera gave me focus confirmation (K-5). The lens itself is fine. on the back-glas its got some minor cleaning marks, is this what could cause the problem? Today i took many pictures of different subjects and played around with the AF-fine adjustment. With a full +10 it seems to work better now but still, especially farther targets are not 100% in focus, while close targets are in focus. i used some text close-up to make the fine-tuning. after shooting targets about a few meters away it seems, if the lens was still front-focusing.
i tried this with the aperture at F/3.2 i didnt try other aperture-setting so far.
Thanks for helping

07-13-2014, 04:21 AM   #2
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The thing is that the AF point is much bigger than the little AF point overlays in the viewfinder. So its possible the camera locked focus correctly, but on something other than what you expected.
But to be safe, you might want to search the forum or blogs for a post about AF calibration and AF charts. You can find some of these charts online for free and you just print them out and do the testing to see if your particular camera/lens combination needs adjustment
Also, the DA 21mm is a very wide angle lens, so if you use f8 you will have a huge DoF, which can mask focusing errors
07-13-2014, 06:32 AM   #3
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QuoteOriginally posted by Na Horuk Quote
But to be safe, you might want to search the forum or blogs for a post about AF calibration and AF charts. You can find some of these charts online for free and you just print them out and do the testing to see if your particular camera/lens combination needs adjustment
Also, the DA 21mm is a very wide angle lens, so if you use f8 you will have a huge DoF, which can mask focusing errors
Targets are a waste of time. Set up the AF correction so that infinity is in focus - take a few shots of a distant contrasty object, such as a tree etc. at different AF correction settings, look at the shots at 1:1 on your monitor and pick the best setting. Remember to defocus between each shot. +10 seems a bit extreme, My DA21 needs +1.
07-13-2014, 09:58 AM   #4
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QuoteOriginally posted by kh1234567890 Quote
Targets are a waste of time.
I might suggest the same thing for infinity focus. Particularly in summer, differences in air temperature/density can play havoc with attempting to get a consistent sharp image if any distance is involved. Remember that the camera does not "know" that it is being focused to infinity. It just attempts to match phase on the AF point. What is most important is to provide an unambiguous, high contrast subject of sufficient detail to engage the AF sensor. I am of the opinion that a flat, high detail target at a distance of about 50x the lens focal length is a reasonable way to assess focus accuracy. If it is possible to improve on the AF result using manual focus in magnified live view, there may be a case for adjustment. Using a target for AF fine adjustment is a bit hit-or-miss, however.

In the past, I have suggested the "DotTune" adjustment method* for actual fine focus adjustment. After spending some time attempting it with the K-3, I can no longer give that recommendation. The MF focus confirm behavior on that camera is too sloppy for DotTune. Things may be different with a K-5, so I won't discount it completely.

What I can recommend and know that it works nicely is the moire' method. The details are on the NorthLight Web site:

AF microadjustment using moire' interference
Canon cameras are used in the discussion, but the technique may be applied to any digital camera. The section titled "Alternative AF setting technique using the Moire technique" works best for me. A single step of fine adjustment is readily apparent using this technique, making it as accurate as any for Pentax cameras. One extremely important thing that is not mentioned is that focus peaking must be turned off to properly use the moire' method!

Steve

* Goggle "DotTune Autofocus"


Last edited by stevebrot; 07-13-2014 at 10:30 AM.
07-13-2014, 03:27 PM   #5
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QuoteOriginally posted by stevebrot Quote
What I can recommend and know that it works nicely is the moire' method.
Been there, done that. The problem being this method is that with something like the DA21 you end up far too close. 50x21mm is just over a metre which will not get the lens reporting the subject distance range as 'distant' to the camera, which is what you want. To get consistent results it is also a good idea to defocus for each go - the near end stop seems to work fine.

Last edited by kh1234567890; 07-13-2014 at 03:35 PM.
07-13-2014, 03:44 PM   #6
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QuoteOriginally posted by kh1234567890 Quote
which will not get the lens reporting the subject distance range as 'distant' to the camera, which is what you want
That is something new to me. What difference should that make?



Steve
07-13-2014, 04:04 PM   #7
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QuoteOriginally posted by stevebrot Quote
That is something new to me. What difference should that make?Steve
According to the fineprint of the various Pentax etc. patents the lens transmits to the camera a range of 'fiddle factors' from its look-up table as well as the subject distance. These are then used by the camera (amongst other things) in calculating how much to move the focusing motor. The DA21 knows about three distance zones - distant (infinity to just a smidgen nearer), close (from there to a couple of feet or thereabouts) and macro (from there to the near end stop). As the phase detect AF method is focus distance sensitive and as the number and size of the focus zones varies between lens models it would make sense to use the one common to all lenses - the most distant one.
07-13-2014, 04:47 PM   #8
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QuoteOriginally posted by kh1234567890 Quote
As the phase detect AF method is focus distance sensitive
That is the part that I have been unaware of and have not been able to find one in the last hour or so of looking. Can you provide a reference (preferably not a casual Internet forum post)?


Steve

P.S. "how much to move the focus motor" relates to focus speed and af prediction, but is not an element of calibration. I can understand the utility for that case, but not the latter.

07-13-2014, 05:51 PM   #9
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hey guys, thank you all for the very detailed posts. i have done a few near infinity shots ranging from +10 to -3 focus adjustments today and i will look at the results tomorrow, as its cose to 3 am here and we just won the world championship, wohoo
just a question, may it be possible that the focus is more/less accurate when near infinity than when focusing on a target maybe 1 or 2 meters away? i will provide more details along with some pictures tomorrow. thank you all again
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