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10-24-2009, 05:18 AM   #1
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focus mystery - lens or camera?

Hi. It meens to me that my camera, Pentax k20d has some back focus issues. I am quite sure that it cant be the lenses, Sigma 10-20, sigma 100-300, tamron 17-50 and pentax f70-210.

The strange thing is, that with any of these lenses at the maximum focal length the focus seems to be at the focus point or slightly behind (camera set zero at focus correction setup). But when I put any of these lenses to the minimum focal length, the focus is way behind the target point, where I am aiming at. The problem seems to be worst with the shorter focal lengt lenses. The focus correction setup at k20d does not seem to be able to correct the problem even set to the maximum correction level.

So the question is - is the problem in my Camera? At the least according to the evidence it is so. Can anybody confirm? I still have warranty on my camera, so I hope its the camera, not all my lenses!

thanks

10-24-2009, 09:25 AM   #2
axl
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can you post some examples of test shots?

Peter
10-24-2009, 10:06 AM   #3
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Preferably test shot using a good focus test chart like the one at focustestchart.com. Many focus discrepancies like what you are describing are actually simply a matter of the camera choosing to focus somewhere other than where you aimed - the AF sensors are much larger that the little red lights that some rely on to tell them where the focus is. And that's why wider angle lenses can appear more problematic: it's less likely that your subject will actually fill the whole AF sensor, meaning it's more likely the camera will see something else in front of behind the subject and choose to focus on that. This isn't a camera defect, it's an unavoidable effect of pentax' steadfast refusal to implement mind-reading technology - or maybe our own refusal to submit to Borg assimilation :-)
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