This won't work unless the M42 lens is machined to sit deeper in the C/Y flange, unless you have no desire to focus to infinity. The lens registration is close but not close enough.
Also, stop down metering isn't for everyone. It's fun for a bit, but in the end, a PITA when really wanting to just go out and shoot.
Phil Forrest
Phil,
Not sure if this is coming from specs published somewhere, but I have used many M42 lenses, Helios, Pentax, Yashica, Mamiya Sekor, Porst, Fujica, and who knows what else, on Contax 139, ST, 159, RX, Aria, and RTSIII, with a variety of different adapters, for years, and there has never once been a combination that didn’t focus to infinity.
And stop down metering on these cameras is also a complete non issue because the viewfinders are so bright. Normally, I won’t put up with stopped down metering for very long either, but it was never a bother on these bodies. Anything from wide open down to f/5.6, focusing and metering and shooting at those apertures is easy to do, and was never a problem for me. Anything more closed down than that, yes, you’d have to meter more open then set the aperture, but most of the time it’s just set it to aperture priority, point and shoot. That small, rare inconvenience, is more than compensated for by the fact that the bodies are so far ahead of any other M42 body one could name in terms of viewfinder brightness, available shutter speeds, shutter speed accuracy and metering accuracy. They’re just more modern cameras, and the registrations are definitely close enough.