Quote Originally Posted by Tom1956 View Post
It's the shutter. Time for CLA, or stay below 500. There's no sane need for these superfast shutters in the first place.
If it is the shutter, it looks to me like the shutter has a curved edge.

I'd suggest putting the multi-stage adapter on with a fairly transparent 77mm (UV or skylight??) filter of similar physical size and then using depth of field preview plus a smaller aperture to see if you can see any vignetting.

Some ground glass at the film plane with an open shutter may also allow you to see what is happening.

The Canon G11 article you referenced deals of course with a camera sensor that is about the size of a fingernail - the conditions that lead to vignetting change with real focal lengths and sensor sizes. It may be that a 35mm lens used with 35mm film needs a "steeper" adapter.