Originally posted by Canada_Rockies There is a lot of confusion on this matter and the behavior appears to differ according to the model of camera and maybe even according to firmware version. I am able to use catch-in focus with all of my lenses with conductive bases on the K10D (firmware v1.20). Those include the above-mentioned Jupiter-9 and is true for that lens even when the camera "thinks" (correctly) that it has no A-contacts. I don't know that this is true for other bodies, though I have read reports of users sanding the paint off their M42 lenses for the purpose of enabling catch-in focus with good success.
As noted above and in my previous comment, the base of my Jupiter-9 is conductive enough to allow catch-in focus and on rare occasions even results in the camera displaying an aperture in the display. This makes no sense in that the A-contact is supposed to "gate" aperture control by the body and does so effectively on my other A-contact body (Super Program film camera). I believe it is due to a bug (ahem...feature) in the firmware.
To muddy the waters, there is a persistent rumor that fully shorting the contacts with foil (pressing the foil into the indent for the "A" contact) will magically correct metering issues on the older K10D and K20D bodies by making the camera think that an A-contact lens is mounted. I remember doing this experiment a few years back and sure enough the "F---" disappeared and an aperture number was displayed instead. What this meant is that I could do green button metering in M mode with the lens wide open and set the lens manually to the aperture on the display and get better accuracy. I cut the experiments short because I was wary of possibly doing an "illegal" combination of contacts and damaging something. (I know...sounds superstitious, but I write software and superstition is a valuable attribute in my profession.) Besides that, the trick only worked with my M42 lenses.
Short summary? I would discourage fiddling with the contacts beyond possibly sanding the bases on non-conductive lenses to enable catch-in focus, assuming that is important to you.
Steve
P.S. I have one of the early Super-Tak 55/1.8 with the thin mount flange. If mates up perfectly to the body flange on my SV film body and barely has enough width to snug up to the body when adapted to k-mount and quite nicely does not touch any of the contacts. It meters the same on the K10D as my later version with the wider flange which means that it meters the same as my Pentax-K 55/1.8...lousy.
Last edited by stevebrot; 07-19-2013 at 11:02 PM.