Originally posted by brothereye I haven't used a K20D, but apparently I have either low standards or an especially good K10D, it focuses fine for me in conditions that are so dim I can't see to focus manually, and it does it plenty fast, at least with the DA* 16-50. That was something that really impressed me about the K10D when I got it, in fact. This doesn't preclude other camera's from having faster AF systems, but so far, I'm pleased with the AF performance of my K10D.
Oh well, maybe I'm just remembering the AF systems on the 35mm cameras I owned(and MF 35mm before that), all those years ago.....
Pentax's AF works fine.
I have been getting sharp pics with no major problem from my Ds, K10 and K20.
Nevertheless, some other makers's AF are better (I have hands-on experience with Canon but testimonies show that it is also the case for Nikon or Sony).
It's not like you won't be able to get sharp shots with a Pentax as no AF is THAT bad but in comparative testing on the same subjects (K20+50f1.4 in one hand 30D + 50f1.4 in the other and shooting the same subjects alternatively with each camera), the Canon was always faster and most of the time more accurate meaning less oof shots at the end of the day.
Make this test: shoot like you did when you had no LCD screen in the film day.
Shoot you subject only once and never review the pic until you download it to your computer.
When I did that with my K10 and my 30D, I was surprised to see I was getting almost 45% of more or less oof shots with the K10 while I almost didn't get any with the Canon (easy subjects in good light).
Sometimes, the difference was small but direct comparison with the precisely focused shot revealed it clearly.
K20D is better but I still get clearly more oof shots with it than with the 30D (now 3 years old).
Pentax will progress in this department but so will other makers that have the advantage of having years of experience in developping pro equipments that drive them forward at a faster pace.
The thing I also realized is that, these small AF errors will quickly destroy the MP advantage the K20 has over the lowly 8MP 30D.
A precisely focused image will always be better than a less precisely focused one, even with double the resolution and even if the focus error is small. Digital is less and less forgiving on the AF precision requirement as the MP count goes up... film days are far away now.
My conclusion so far: it's not slower focus that can hinder Pentax from being the system it deserves to be, it's the imprecise focus.
It's a shame because at equal focus precision, the K20 pics are quite a bit better than the 30D ones.
People who claim that Pentax is slower but more precise than Canon must have got different gear that I have...