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10-18-2011, 06:11 AM   #1
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Prime lenses AF adjustment

I have a question regarding AF accuracy of prime lenses when shooting objects that are closer vs objects that are further away. I don't know what to think of a lens which doesn't need any AF adjustment when shooting at MFD, but it needs a helluva lot when shooting objects that are more distant - +10 with objects at approximately 3 metres away (close to infinity) and like +15 at infinity. Could this be a matter of misalignment of elements?

10-18-2011, 06:34 AM   #2
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QuoteOriginally posted by 1marek Quote
I have a question regarding AF accuracy of prime lenses when shooting objects that are closer vs objects that are further away. I don't know what to think of a lens which doesn't need any AF adjustment when shooting at MFD, but it needs a helluva lot when shooting objects that are more distant - +10 with objects at approximately 3 metres away (close to infinity) and like +15 at infinity. Could this be a matter of misalignment of elements?
I am glad someone else (you) posted about this and explained the situation better than I did here:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/162500-da-15mm...us-issues.html

but you will find some very interesting answers / solutions in that thread of mine.

I have a DA 15mm Ltd lens which I find rather tough to fine tune (AF) on the K7, but for some reason it is perfectly in focus with the K5, with no AF adjustment. Go figure!

I am now testing this same lens with my old K20D and it looks that it won't need as much adjusting as with the K7.

I hope you will find a way to fine tune your lens too.

Cheers.

JP
10-18-2011, 09:30 AM   #3
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Thanks for the tip, I completely missed your topic, cr**py PF search engine....
After reading advices they gave you, first thing that comes to my mind is to maybe dial in +10 in order for the lens to work better at infinity but this would be just a lousy compromise... 'cause my logic tells me that you would take more pics at or near infinity with a 15mm lens than with a longer lens. Correct me if I'm wrong.

When I first tested the lens+body combination with focus test charts, I put in +3 and thought that would do the job... seems logical since chart testing is done at distance of approximately 2 feet... So, bang on at MFD, +3 at 2 feet and +10 at infinity.

Has anyone else had this problem and then send it to Pentax for calibration? I'm wondering if such a body/lens can be tweaked to AF properly on both ends of the distance scale.
I haven't done any hard core scientific focus tests on tripod (except with the focus charts) but it is bluntly clear that AF is not working as it should, and since there is this progression of AF ailment, it seems to me that body may be the culprit. What do you guys think? Plese feel free to chime in.

Oh, I almost forgot, the gibbled combination is K-5 + FA31
10-18-2011, 09:32 AM   #4
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I still need to do some tests with the k-r to confirm the ''body's the culprit'' theory but to me it seems a good theory.
Anyway, I would like to resolve this because I use the lens almost exclusively on a K-5

10-19-2011, 11:48 PM   #5
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Some lenses need to fine tune AF, some not. Same lens on different bodies may and may not. Mine took some time to adjust, and remember to note down lens-body combination and adjustment value. It's easy to erase them all.
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