Hi Sven, That's interesting to me. It can help understanding strange behaviours when autofocusing some lenses. Thanks for sharing.
Dario ----- Original Message ----- From: "keller.schaefer" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, June 24, 2004 8:49 AM Subject: FA 4/28-70 - something for the technically minded > 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 > > >
