Originally posted by jfdavis58 I think the correct term for this effect is light-fall-off, not vignetting. Every lens suffers from light-fall-off approaching the edges to some extent but as you can see, special circumstances must be present before it become obvious.
I think John has it right. I've seen this myself with a few lenses in similar conditions. I might suggest that removing the hood may help a little and it doesn't do much in these situations anyway other than to protect the lens from a bump.