Originally posted by Fogel70 BTW, on M42 you do not need to use green button stop down metering.
As the camera is not involved in stopping down M42 lenses before exposure, you can just as well use aperture priority with them.
On M42 lenses you will have to switch between wide open aperture and stopped down aperture manually before exposure, so the meter will adjust to that.
I've made this mistake … as recently as a week or so ago, believing that "Green Button metering" in "M" mode was the same as Av with an M42 lens fitted … it's not, certainly not in every circumstance, in my experience.
I'll agree that, in theory, it should all work, and often does, but, just occasionally the Av metering "gets it wrong", sometimes by a stop or more, whereas the Green Button never lets me down.
Maybe it's my technique, maybe it's my camera(s), but I'd not recommend relying on Av when using a stopped-down M42 lens.