Been researching this lens, and the low-light capabilities are tempting. That said, I'm reading a lot of mixed things about how this lens functions in practice on the C100. Is vignetting an issue? There's no IS on this lens... has this proven to be a problem on the C100? Is this lens fully useable on a C100 at 1.8? Any general experiences from users with this camera/lens combo would be helpful. Thanks!