I use a Zuiko 50mm lens from my Olympus OM camera on my digital camera and it exposes just fine. I have to set the aperture manually in Av mode and then let the EOS camera figure out the shutter speed. I haven't tried it yet in one of my EOS film cameras but it should work just the same.

I have also used my Opticron birding telescope attached to my EOS camera and that exposes just fine as well.