So I have been playing around with comparing lenses in somewhat controlled situations to see if I can detect much difference. I need to work up a better testing scenario as I was not smart enough to fix the focus point for all the different lenses so focus and potentially metering could be different. Oh well I will work on it some more when i get bored again I guess.
Both images were taken at 30mm, f4, 1/60, 580ex II. Both are L Glass of varying quality. Both are straight out of the camera with only crop and sizing.