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05-20-2014, 12:12 PM   #1
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Join Date: Sep 2012
Location: Binghamton, NY
Posts: 23
DA 16-45 uncorrectable back focus *only in middle range*

Hi all,
I've been doing some focus testing on suspicions that my 16-45 might need adjustment. I found that my previously selected fine adjustment of +5 was inadequate when shooting at 28mm.

I found that at BOTH 16mm AND 45mm, a correction of +7 or so is adequate. However, at 28mm even with a +10 correction I still get significant back focus.

I did my tests in moderate indoor fluorescent lighting, at a distance of about 10 feet.

Any suggestions?
(Lens was purchased used from KEH almost a year ago, return / warranty is not an option)

Thanks.

05-20-2014, 12:46 PM   #2
Pentaxian




Join Date: Mar 2010
Location: Frankfurt am Main
Posts: 1,045
You don't mention which camera body you were using (and it is not in your member profile, either). And you also don't tell how you did focus. Was it manual focus, autofocus, or contrast autofocus (LiveView)?

If you were using a K-5 and autofocus without LiveView, the problem may be related to the inconsistent AF of this camera in fluorescent light (depends on frequency mix of the llight). There are multiple threads about this subject from the time short after the K-5 was introduced. This problem could only be fixed by hardware mods, not firmware. It was cured with the K-5 II and K-5 IIs.

In any case, you should try it again with natural light.
05-20-2014, 12:51 PM   #3
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Join Date: Sep 2012
Location: Binghamton, NY
Posts: 23
Original Poster
Oh, sorry. Camera body is a K-5. I don't have any other bodies to test with.

I did my testing from a tripod, using multiple methods:
Focusing using phase AF then switching to live view to check focus accuracy
Focusing in live view to actual optimal focus and then checking to see if phase AF would focus confirm
Focusing using phase AF and eyeballing focus
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*only in middle, 28mm, adjustment, correction, da 16-45 uncorrectable, focus, focus *only, k-mount, pentax lens, slr lens, uncorrectable back focus
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