I purchased a c-mount to EF adapter on whim just to see how it would perform. I've read mixed reviews on this forum about people using adapters. My problem is that the adapter doesn't seem to thread into the mount as nicely as I would like. I can get about 3 solid turns on it until it feels that it might strain the threads on the camera. Comparatively, my camera lens that came with the camera has no problem threading into the mount.
I tired out my canon lens's and was surprised to see that they performed almost as a macro lens. they appeared sharp, but I couldn't adjust the focal is only 3 to 4'. I used a 50 and a 85mm for my test.
I'm guessing this is due to the fact that I didn't rotate my adapter all the way into the mount. It looked like that I had about another 2cm to go until it was fully installed.
here is the adapter I purchased from Amazon: