This page discusses the Live View vignetting problem seen on the 5D Mark II, a problem which apparently affects only non-chipped lenses used with lens adapters (that’s a plausible theory at this point in time).
This issue came to my atttention via several readers. See the discussion thread on the issue.
Based on observing the 5D Mark II behavior taking an exposure without a lens on the camera, the theory that this is a mirror problem seems unrealistic.
The Live View vignetting problem appears to be peculiar to the new Canon 5D Mark II, and to affect any lens with a lens adapter.
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5D Mark II + Voigtlander 40/2 Ultron + CameraQuest lens adapter @ f/2