[stunsworth]

Jo, I had exactly the same problem with my R adaptor on my 5D. This didn't appear to be an issue with the other Canon digital SLRs that I've owned - 300D (digital rebel) and 20D.

As the aperture gets smaller the camera tends to overexposure. You can test this by taking the same shot several times stopping down the lens as you do so. You'll notice the image getting brighter as the lens is stopped down.

It appears that this is due to there being no electrical linkage between the camera and the lens to indicate the lenses maximum aperture. On the pro bodies that isn't an issue as they allow the photographer to manually tell the camera what the maximum aperture is. Not the case on the 5D.

As a compromise I used to dial in an exposure compensation of -2/3 or3 of a stop. I found that gave good exposures at all apertures.