The Shadow Bias doesn't seem to be having an effect. I put it at 0 and at 40 (as high as it would go; I'd typed in 100), and they look the same to me. It's also present whether I render with shadows or not. I don't have a good feel for how thin the lens gets at the edge; maybe it's just too small to work? I only have one light (out of three) casting shadows; do I need to do anything to the others?