I was initially very disappointed with this lens when I got it. Why would you buy an f1.4 lens at this price, when the 50/1.8 is available at its ridiculously low price? Yes – to be able to take low light shots below f1.8 of course… Right ?
My experience with a 1DmkII (1.3x crop) is that it is meaningless to use it below f1.8 or so. It is terribly soft and the focus gets overall suspicious. However, after having started to enjoy the pleasure with really fast lenses such as the 35/1.4L and the 85/1.2L, I felt the gap between these awesome lenses and decided to pick up the 50/1.4 again.
As the 35/1.4L is really good even at f1.4 and the 85/1.2L is stunning at f1.2, you get a bit spoiled. However, the 50/1.4 starts to perform nicely at f1.8 and I have finally understood that it is a stellar performer from f2.0. I mean a really good performer, where it really blows away an otherwise good zoom like the 24-70/2.8L.
Bottom line: I would say that the 50/1.4 is really a “50/2.0L”, as it starts to perform according to the expectancy (given the premium over the 50/1.8) at that aperture…