In preparation for my upgrade to a 5d3 (from a 60D) later this year I'm trying to compile a list of potential prime lenses to get between the 35 and 100mm range. I've already got a 24-105L and will sell my non-L 70-300 to get the L version. Also have a 50mm 1.8 so I'll need to get a 85 1.8 to get my portrait capability back.
Then the question remains: Do I keep the little plastic toy (which never use wider than 2.8 anyways) or just get a pancake (which is actually pretty usable wide open, vignetting aside)? Do I even need a 50? I know the 50mm 1.4 isn't great wide open either and has fragility issues but at least it'll work with the automatic CA corrections in the 5d3 (I shoot RAW+jpeg). The sigma 50 1.4 is...really big for a 50mm prime, has AF quality control issues and won't work with auto CA corrections. Rumor mill has it that the sigma 50 is due for a rebody to the "art" line they've got going now but probably won't get an optics refresh... Oh yeah, the 50mm 1.2 is a no thanks I'll keep the money. Everything 50mm seems to be a compromise and you'd think after so many decades of people using such a prolific "normal" lens the designers would have perfected the formulas by now.
Do I even need a 50?
Well, 40mm in comparison is much closer to 35mm than to 50mm prime and if such angle works for you, then you should consider the new Sigma 35/1.4 ART.
Perhaps, at f/1.4, Sigma 50 is the best prime lens at the moment (at least in sharpness and bokeh quality). Canon's 50L has similar AF problems (focus shift), so (IMHO) it is not a much better solution.