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05-09-2010, 03:00 PM   #1
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Auto focus adjustments

I had a feeling that my K100D was back focusing with the kit lens, and after a few different tests, I determined that it was. So I used the debug menu and found that an adjustment around +200 seemed to correct the issues. Now the problem that I have is that my Pentax M 50mm 1.7 front focuses while using the catch in focus. Obviously this problem makes sense, but I'm wondering if the issue is with my body or the DA 18-55mm.

I do not have another AF lens to test with. I'm probably not going to send anything in to Pentax because I bought the K100D fairly cheap, and I would use the "repair" money towards another body if that is the case. I know that the K100D doesnt have the option to make an adjustment based on each different lens, so any adjustment I make will have the same effect on each lens.

I guess my question is, is it worth picking up another used kit lens to test or is my K100D the issue in this case?

05-09-2010, 09:46 PM   #2
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There's almost no chance that the kit lens could be backfocuing - it's incapable of shallow enough DOF to exopose the small discrepancies that one might typically see. So I'd start by re-examining your test methods. Did you use a bona fide focus test chart, carefully follow the isntructions, and test using good (not incandescent) lighting? Could you post the results?
05-10-2010, 09:47 AM   #3
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I used this chart http://www.k10dbook.com/newchart.pdf and at around f3.5-4 the top half of the chart will be in focus and the bottom wont be at all. I will post some images in a few. I also used the sun to my back with good lighting.
05-10-2010, 10:04 AM   #4
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I left these as big as flickr would let me.

no pp done, and these are the consistent results. f4, 1/30s with tripod and 2 sec timer.

before


after making +200 adjustment


maybe its user error but I have been noticing soft focus, which is why I tested it.

05-10-2010, 10:30 AM   #5
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AF microadjustment for the 1Ds mark III, 1D Mk3, 5D Mk2, 7D

I've had pretty good results with this focus test because it's less prone to errors due to angles and miss-focusing. I used it to establish my baseline, then fine tuned the results with other focus tests. For the most part I backed 2-3 clicks off of the northern light values, since they caused slight front focus under artificial light.

Edit: To answer your original question, it's probably the lens rather than the body. My 16-45 f/4 and sigma 30mm f/1.4 back-focused significantly, while 10-17 fish and 50mm f/1.4 required almost no correction.
05-10-2010, 11:27 AM   #6
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Wow, that's rather more severe than I'd have imagined. Although I do think you overdid the adjustment - it's normal and correct for there to be more in focus behind the target than in front at that focal length. You've actually made it the other way around.

I'd google around and see if you can find instructions (I know I saw them long ago, probably on dpreview) for making a hardware adjustment to the AF sensors yourself - something about some little screws on the underside of the camera,ten decide if that seems worth a shot.
05-10-2010, 11:43 AM   #7
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would this be it? I have fixed my front focus problem - how to [Page 1]: Pentax SLR Talk Forum: Digital Photography Review

I may try this once I'm done with my finals.
05-10-2010, 02:23 PM   #8
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figured i needed a break from studying, so I decided to open up the thing and adjust the screws. this is with no software adjustment.



seem more accurate?

05-10-2010, 07:55 PM   #9
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Good job finding that article and performing the procedure! Definitely looks better, but of course, the open question is how this will affect other lenses.
05-11-2010, 08:28 AM   #10
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My only other lens, 50mm M 1.7 is fine using the focus confirmation in the view finder as well.
05-11-2010, 12:40 PM   #11
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I'd still want to see how it affects another AF lens - without knowing *exactly* how all this works, it is definitely possible that the focus indication and the actual behavior of AF could be out of sync. But I guess no need to worry about that until you get another AF lens.
08-19-2010, 03:02 PM   #12
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QuoteOriginally posted by Marc Sabatella Quote
... it's normal and correct for there to be more in focus behind the target than in front at that focal length.
The closer you get to the subject, the more the distribution of the DOF approaches 1:1 (before : beyond).

I've added that to my AF adjustment hints.
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