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11-09-2009, 07:29 AM   #1
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AF Calibration K20D & Tamron 70-200 f/2.8

I recently purchased the Tamron 70-200mm f/2.8 lens for my K20D. I'm noticing what may be a slight back focus problem with a lot of shots (most notably at 200mm f/2.8). In the example, the focus point should have been on the runners. Of course, it could easily be my error as well.

I have three questions...

1) Does the calibration work for non-Pentax lenses (does it store the data for that specific lens)?

2) I downloaded the calibration charts from pentaxdslrs.blogspot.com. What zoom setting on the lens should I test? The midpoint (135mm) or one of the extremes?

3) What does the Pentax calibration change, the gain or the bias?

Thanks,

Kevin


Last edited by klh; 11-10-2009 at 06:21 AM.
11-09-2009, 09:14 AM   #2
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Wondering myself. Having the same issues. Also, just purchased...so it might just be me.
11-09-2009, 03:06 PM   #3
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QuoteOriginally posted by klh Quote
I recently purchased the Tamron 70-200mm f/2.8 lens for my K20D. I'm noticing what may be a slight back focus problem with a lot of shots (most notably at 200mm f/2.8). In the example, the focus point should have been on the runners. Of course, it could easily be my error as well.

I have three questions...

1) Does the calibration work for non-Pentax lenses (does it store the data for that specific lens)?

2) I downloaded the calibration charts from pentaxdslrs.blogspot.com. What zoom setting on the lens should I test? The midpoint (135mm) or one of the extremes?

3) What does the Pentax calibration change, the gain or the bias?

Thanks,

Kevin
That big of a difference is not FF/BF. Looks like you let the camera choose the focus point and it chose the hill in the back. Or you have select focus point on one of the top points.
11-09-2009, 03:18 PM   #4
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Would have to agree with enoeske. With no other evidence of BF, it would seem to be due to focus point aiming at the grass behind. Kevin, do you have any other examples of this BF? Any under more formal testing conditions?

Indeed major BF/FF is not a problem the camera's AF adjustment can fix - it's for minor BF/FF.

11-09-2009, 04:33 PM   #5
klh
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QuoteOriginally posted by Ash Quote
Would have to agree with enoeske. With no other evidence of BF, it would seem to be due to focus point aiming at the grass behind. Kevin, do you have any other examples of this BF? Any under more formal testing conditions?

Indeed major BF/FF is not a problem the camera's AF adjustment can fix - it's for minor BF/FF.
I have other examples, but also some where focus looked better, and others where I clearly missed the focus. Now that you mention it, I may been using the multiple focus points for that picture. I'll try to do some controlled testing and let you know. Thanks.

This camera is my first foray into autofocus, so I'm still learning. I haven't had many problems with the kit lens, but with the smaller aperture, it's hard to miss.
11-10-2009, 04:35 AM   #6
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Well, it looks as if the main source of the trouble is the photographer. On a tripod with the focus chart, it appears to be accurate at both 70 and 200mm. If anything, it's slightly front focusing, but the target is still in accurate focus. I'm going to leave the settings alone for now and practice, practice, practice.
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