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10-21-2009, 11:05 AM   #1
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K-m autofocus problem. What could this be?

I have noticed that pictures taken with my K-m and the FA 31 Ltd often turn out soft. After performing some quick'n'dirty testing it turns out that the camera tends to slightly front focus near the minimal focussing distance (focal plane around 1-2 cm off) and to randomly but significantly back- or front-focus at distances of about 1 metre (focal plane about 10 cm off). I could not yet verify that this happens in any other lighting than lowish tungsten lighting, nor that any of my other lenses show this problem.

Does anyone have an idea what the problem could be? Is my lens or camera defective or is this simply a limitation of the K-m's AF engine? I am about to upgrade to a K-7 or K-x soonish so if possible I'd like to avoid sending the camera and lens in for AF adjustment - unless there is a chance that either of them are actually broken. Any advice would be much appreciated...

10-23-2009, 10:40 AM   #2
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U can use hacked debug mode to adjust the camera's focusing, also test it with one of those focus test card.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/67897-pentax-firmware-hack-site.html
10-23-2009, 10:46 AM   #3
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It's unfortunately normal for a DSLR AF system to misfocus a little in low tungsten light - the phase detect AF system doesn't work so hot when the light spectrum isn't what it expects. Some cameras - like the K-7 - have a special sensor to notice when the light is esepcially yellow, so the AF system can compensate. I don't know that I'd ever expect it to be 10cm off, though. Have you tested this with an actual focus chart to b sure the camera hasn't simply chosen to focus on something in front of behind what you wanted it to? I assume you're in center point mode so you have *some* control over where the camer focus, but it's not pinpoint control - the center focus point is pretty big, and can focus on anything it wants within its zone (about the size of the inner bracket you see in the viewfinder).
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