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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