Originally posted by Miserere Unless there's an 'A' setting on the aperute ring, you will only be able to use them in M mode with stop-down metering.
As I understand it, "stop-down metering" means there is no auto-diaphragm, and you have to meter from the lens being stopped down. This is opposed to metering from wide open and having the camera stop down upon pressing the shutter via the auto-diaphragm lever.
If my understanding is right, I believe you're wrong, Miserere. As long as the lens has an auto-diaphragm lever (many of the M series and third party lenses do) you won't have to do stop down metering. That makes for a much brighter viewfinder when the iris is set smaller than wide open.
I can't speak to the K10D, because I don't have one, but if the green button behind the shutter works like the one on the K200D and the K20D, here's a little trick while shooting in Manual mode (I never tried it in other modes). Go into the menu, set the green button to adjust shutter speed for correct exposure when pressed. As long as the lens has an auto-diaphragm lever (which many lenses older than the A-series have) the camera will grab that lever, stop it down, set the shutter for correct exposure, then release the lever again. Now you're correctly exposed and ready to shoot. I used to do this all the time on a K200D and K20D with a K mount Takumar Bayonet that had no A setting on the aperture ring.
I should mention auto-diaphragm is the same as auto-aperture, and Steve's remarks are very informative to that end.