Originally posted by Kenn100D Does it mean that Phase AF of K-5 being accurate is dependent on how the lens was calibrated from production?
Frankly, I don't know.
Theoretically, a lens shouldn't have to be calibrated as much as the body. Because the AF module anyway sees any defocus.
But I didn't systematically check it.
From two K-5 bodies and two lenses (FA31 and DA70, i.e., four combinations) I'd say that there is significant sample variation between bodies and some bodies need less AF correction than others. When needing less AF correction, K-5 phase AF often beats contrast AF accuracy. With the lens being the smaller factor. But not always.
The best focus I achieved at 31mm at f/5.0 was 1.35px with contrast AF while the two best phase AF measurements I got were 1.24px (which is a fabulous value) and 1.45px. That's all in one ball park.
I don't have enough data to isolate factors which make phase AF the option to go for. I am sorry.
But let me say this much (which is the safe variant of statement I can make):
With the K-5, phase AF can be more accurate than contrast AF. But the performance is more fragile and in other cases, contrast AF may win because it delivers a more stable accuracy (more stable across bodies, lenses and apertures). OTOH, contrast AF can fail miserably and fortunately by so much that it is visible to the naked eye.
Note that the above is a non trivial statement. Many testers assume that Contrast AF or Manual LV focus is the way to go for tests. Not necessarily so though with the K-5.
Note:
I said several times that I consider mechanical focus precision, in particular a play (or better its absence) between the forward and backward focus direction, to be crucial for accurate focus. At least for focus algorithms which go forth and back. Most contrast AF algorithms do it while it can be avoided in many cases for phase AF. But better contrast AF algorithms (better than Pentax uses) can avoid this too, I worked on them. So, if a lens lacks mechanical precision engineering, it may face some focus accuracy problems for reasons other than optical. So, the build quality may actually pay off here. After all, top notch focus accuracy is much better than the thickness of a hair.