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03-15-2011, 09:25 PM   #1
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Focal plane changes with aperture.

I am testing yet another DA* 55/1.4 lens. (See my other thread on this topic.) I also have a brand new K-5, which seems like a big step up from my K-7. At any rate, I have noticed that the AF progressively backfocuses as the aperture is reduced from f1.4 to f5.6. I do not believe that this is a case of focus shift, as the backfocusing does not occur with manual focusing, that is, if I obtain sharp autofocus at f1.4 and then put the lens on manual focus (so that there is no further adjustment in the focusing mechanism,) stepwise reductions in aperture do not result in shifting of the focal plane. It only occurs with autofocusing prior to each exposure.

Has anyone else had this experience, and can anyone explain it? My other lenses do not exhibit this behavior.

Rob

03-15-2011, 10:06 PM   #2
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Maybe the AF system is trying to compensate for focus shift that is not there Sounds like you nailed it down to an AF issue.

If you get the same result when repeating the experiment several times, then you should return the lens for an exchange. If this happened to you before with another sample, it might be worth to contact Pentax for an explanation.
03-15-2011, 10:25 PM   #3
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Do you see the same behaviour with both cameras?
If you change the f-ratio on the camera and trigger AF again do you see the lens moving?

Is the focusing of the 55/1.4 not erratic this time?
03-16-2011, 08:50 AM   #4
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Oh no! I just checked for sensor stains and found two in the center of the frame. They cannot be removed with pressurized blower. Probably not a big deal in actual use, but I am sending the camera back.

On my K-7, the lens focuses very erratically--sometimes front, sometimes back. I do not seem to be destined to own a DA* 55/1.4.



Rob

03-16-2011, 01:30 PM   #5
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Don't send it back, instead send it in for service.
03-16-2011, 11:42 PM   #6
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Ray Pulley posted an observation regarding inconsistent focus in the presence of AF adjustments with the K-5. Not sure if this is related to your problem, but you could have a go with AF adjustments reset just to see what will happen.

Is the focusing only erratic with the K-7 or with the K-5 as well?
03-17-2011, 01:07 AM   #7
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I have experienced the same thing but with different lens. It was Sigma 100-300/4. Wide open it was backfocusing but from f5.6 it was OK, and past f11 it was front focusing. Of course I returned it after 2 days out....
03-17-2011, 09:00 AM   #8
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QuoteOriginally posted by Class A Quote
Ray Pulley posted an observation regarding inconsistent focus in the presence of AF adjustments with the K-5. Not sure if this is related to your problem, but you could have a go with AF adjustments reset just to see what will happen.

Is the focusing only erratic with the K-7 or with the K-5 as well?
In my case with a brand new K-5, I installed firmware v.1.03 immediately, so there was no opportunity (or desire on my part) to perform a comparison test. However, my experience does not inspire confidence in Pentax's AF system or in the DA* 55/1.4 lens. On my K-7, that lens focuses almost randomly around the focus target, and this is with 5 different copies. None of my other lenses has that problem.

I had to return the K-5, because of centrally located sensor stains. At this point, I am unsure about trying another K-5 in about a month or waiting for its successor. Who knows when that might be released and what bugs it might have? Although I am feeling frustrated, I do recognize that my current kit can produce stellar results.

Rob

03-17-2011, 09:02 AM   #9
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QuoteOriginally posted by axl Quote
I have experienced the same thing but with different lens. It was Sigma 100-300/4. Wide open it was backfocusing but from f5.6 it was OK, and past f11 it was front focusing. Of course I returned it after 2 days out....
That is the reverse of what I experienced, but it does reveal a defect in the AF/lens interface.

Rob
03-17-2011, 08:52 PM   #10
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QuoteOriginally posted by robgo2 Quote
At this point, I am unsure about trying another K-5 in about a month or waiting for its successor. Who knows when that might be released and what bugs it might have? Although I am feeling frustrated, I do recognize that my current kit can produce stellar results.
I hear you, this is frustrating.
If I were you, I'd try a focus chart (instead of the LensAlign) and perhaps different lighting conditions (unless you are using bright daylight already) just to see whether it makes a difference.

Seems hard to believe that you only got dud 55's, given that you tried different vendors, but maybe there was a large batch of them which are lacking somehow?

QuoteOriginally posted by robgo2 Quote
That is the reverse of what I experienced, but it does reveal a defect in the AF/lens interface.
The Sigma 100-300/4 has a great reputation among Pentaxians. I reckon Peter got a copy with an optical defect or a lens chip problem.
03-18-2011, 05:00 AM   #11
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QuoteOriginally posted by Class A Quote
I hear you, this is frustrating.
If I were you, I'd try a focus chart (instead of the LensAlign) and perhaps different lighting conditions (unless you are using bright daylight already) just to see whether it makes a difference.

Seems hard to believe that you only got dud 55's, given that you tried different vendors, but maybe there was a large batch of them which are lacking somehow?


The Sigma 100-300/4 has a great reputation among Pentaxians. I reckon Peter got a copy with an optical defect or a lens chip problem.
I agree on all accounts
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