The sigma and nikon at 5.6 look very close. Extreme corners look soft in both cases but that seems more the subject matter and distance. Does anyone have comparison of corners on a flat surface between these two. I've been looking into the nikon 1.8g for a 50. I tend not to shoot wider than 4 or 5.6 anyway, seems like $700 would almost be a waste seeing the nikon is now $196 on sale.