Originally posted by Ira Now I'm confused.
All I shoot are M42 set to M, and I still don't understand this noise I'm supposed to hear.
That would be because for that nothing has to move in the camera: with manual aperture, the lens is *always* stopped down to where the ring is set, you'll just be reading whatever's actually hitting the meter in there: if you turn the ring to f8 it's the same as shooting an F8 lens as far as the mechanisms are concerned. With (pre-'A') K-mount lenses , things can and need to stop down for metering.
On my K20D, I actually rarely even use the green button: I was raised on stopdown-metered cameras like Canon FTs, so it's natural for me to move a lever (configured for 'optical preview' ) to stop down and meter, and then match (Well, let's say 'consult'
) the little display like the old needles. If you hit the green button, the camera tries to stop down any lens on there and sets the shutter speed the meter suggests. The sound one hears in either case is that mechanism being actuated.