don't know why, but you're not the only experiencing this if you googled:
When mounted on Pentax K-x, it works as a Pentax K manual lens. I have to use green button for stop-down metering. There is the undocumented observation about K-x in working with green button on K lens, it has an extra mirror slap that goes with stop-down metering operation. So far, I have not found a clue why the mirror slap is required in the metering.
same guy asked the question in pentaxforums, got this answer:
The sound you hear is not the mirror flipping but rather the aperture stopping down. It gets dark in the viewfinder while stopped down, and makes noise. Using the green button you get a quick stop down, darkening the viewfinder, followed by an immediate opening of the aperture. If you watch carefully you will see that the subject is always visible. Try it at f/5.6 and you will see that it just darkens briefly.
thanks and I've read those posts before asking the question here because my concern is that my camera mirror sometimes do flap, sometimes doesn't during stop-down metering. Since yesterday, if flips on 3 occasions, it doesn't most of the time (I've tried on many apertures).
theres is also one posts you mentioned above with the following comments (which seemed to imply that the mirror is not supposed to flip?)
Hello Hin Man and thanks for your work
Tried today my M42's lenses on Kx and the green button doesn't work as contrary as with the "K" or M lenses. I use a real Pentax ring. Everything is ok with the *ist Ds. Do u have an idea of what's going on ?
Hin Man said...
@Philip, first thing to check is ensure that you have not assigned Green button for another purpose such as optical preview or other function. In M mode, the (+/-) Av button serve the same purpose as the Green button. Give that a try also. Unlike K or M manual lenses, you will not hear any close of aperture blades as the lens is already stopped down to the set aperture. You note the changes in the shutter if the lighting scene change. It is easy to get confused when the shutter does not change when shooting scene are similar resulting in identical shutter speed.
Thanks for your quick and nice answer. You were right. The lack of noise made me worry, but it works. Not so good as with the *ist Ds in my opinion.
well, i haven't actually used any M lenses other than the lensbaby (which is not really an M lens but works like one).. and i did not use it on manual mode, i used aperture priority all the time.. so can't help you there.
i think a lot of people in the links i posted have mentioned that the previous incarnations of pentax cameras don't seem to do that with M lenses either.
perhaps you can mail pentax about this - then they might correct it in firmware subsequently.