Originally posted by grahame using M42 on pentax lens does not need pushing green button in M setting. When you turn the aperture ring, you have stopped down the lens already. M42 lens can work under A setting and camera can find shutter speed for you.
No pentax DSLR can stop down an M42 lens. If the lens is an "Auto" lens without M/A switch, you can modify the lens by fixing the pin to make it a stop-down lens.
You use the Green button to either stop down and METER or, as with M42 lenses and K-mount lenses without an aperture ring with auto setting, just to METER after YOU have done the stopping down. More modern old lenses have an auto setting on the aperture ring that pushes a pin in the mount and tells the camera that it is has auto aperture. This makes the lens work as any new lens and you select aperture and shutter speed on your dials. The only thing that is different is that you lack the auto focus.
EDIT: When in M-mode this is true, you need the green button but if you put the camera in Av-mode it does the job for you. I did not understand your reply at first.
EDIT2: I have now played with all my older lenses and can conclude that all of them works differently when in Live View and also different from time to time which is a little anoying. All due to how the contacts are shorted I guess.