I have a Zeiss Planar T* 85/1.4 (made in W Germany) that I got along with a Contax 139Q. The 139Q is not in working order, but the lens is in fantastic condition. There is also a Mamiya m645 and a Pentax 645 available. I am wondering if it is possible to get an adaptor for the Zeiss for use on either the Mamiya or Pentax?
I also currently have a Canon DSLR (500D). Is there an adaptor to use the Zeiss lens on this camera as well? Where would I find the adaptors? I have heard of something called "stop down metering". Can anyone help me understand that?
The Zeiss 85 is made for 35mm film, the Mamiya 645 and Pentax 645 are both much larger. If you were to find an adapter to fit one of those bodies, the result would be grossly vignetted and not worth the trouble.
On the other hand, there is an adapter for C/Y mount to EOS bodies. I have one, but there's no name on it. You can search ebay and amazon. It's a plain piece of machined steel, probably cost me $20.
Using one manufacturer's lens on another's body presents difficulties. The chief among them is the lack of lens aperture control by the body. Stop down metering sets the camera so that it responds to you rotating the aperture ring. You set the aperture on the lens, and the camera (hopefully) calculates the correct exposure. Your best bet is to google the phrase 'stop down metering'; there's lots of discussion on the subject.
Frankly, if I had your lens, I would try to find a working Contax body, such as an Aria. Unless you want to sell the lens; you could get several hundred dollars for it on ebay.