I just found something interesting (at least for me) when trying to fix a FA 4/28-70 lens that wouldn't autofocus properly. On focal lengths longer than 50 it would not stop at precise focus but run past it and back the full travel, then stop, blinking 'unable to focus' in the finder. It did that on several Pentax AF bodies, so definitely the lens was at fault, not the body. On shorter focal lengths, everything was fine. Using the *ist D I found that the reported focal length was completely wrong for 50 and above (e.g. 31mm was reported for actual 70mm), so I took the lens apart and cleaned those contacts on the barrel that report the position of the zoom ring. Now both the transmission of the focal length and the AF work well again.
So apparently, the the AF system takes into account the focal length, when calculating how much it has to displace the lens (or parts of it) to achieve focus. Somewhere in the lens the displacement/distance curve for each focal length must be stored, so when the AF system determines that the focus needs to be corrected from say 10m to 5m, the lens reports back "as I am set to 50mm, you need to turn the AF shaft x turns... ...or something to that effect. Sven
