I just finished up several weeks (evenings) of testing 3 50mm lenses; Sigma 50 f/1.4, Zeiss 50 f/1.4 and Zeiss 50 f/2 MP. I shot the same scene with each lens on both my 5D and 1D IIn bodies. I also shot the 24-70 and Tamron 28-75 at 50mm to compare them to the primes. After a bunch of testing, here is my summary of the 50's.
Sigma 50 f/1.4 - Sharp enough wide open in the center. Corners were noticeably soft. Bokeh was buttery smooth. AF accuracy was questionable. Sometimes it was spot on, other times it was off. Manual Focus was gritty and somewhat sloppy. At f/2, the sharpness & contrast came up but corners were not "fixed" yet. Bokeh at f/2 was good but not as thrilling as f/1.4 At f/8, the corners remained soft. There was some focus shift, esp. between f/2 and f/2.8. The focus plane shifted away from the camera. For me this lens is one to shoot at f/1.4 or f/2.
Zeiss 50 f/1.4 - Dreamy & lacking contrast at f/1.4 and the bokeh wide open was annoying. At f/2 it is a different lens. It was contrasty, and sharp. Sharpness equalled the Sigma at f/2. Bokeh equalled the Sigma. Corners were sharper and colors were better (slightly). At f/2.8 through f/8, the Zeiss was sharper than the Sigma and the corners were noticeably sharper. The Zeiss suffered from some pretty large focus shift from f/2 thorough f/2.8. If you want close up shots at f/2.8 to be in focus, you need to focus a bit ahead of the target. Overall, for f/2 through f/8 use, the Zeiss was the one to have.
Zeiss 50 f/2 - Center sharpness is great at f/2. Corners are dark and somewhat soft. Bokeh is pleasingly smooth. f/2.8 Cleans up the corners a bit but they are still soft. By f/8 the lens is outstanding across the field. Sharpness is top-notch both close up and at infinity. Floating elements prevent the focus shift that the other fast 50's had so getting sharp pics at 3' is much easier. The big drawback to this lens, other than price, is the vignyetting at f/2 and the somewhat soft corners at f/2. At f/2.8, the lens is just plain sweet.
All there primes were better than the zooms at 50mm. Corners were equal or better. Contrast was equal or better. Color was equal or slightly better.
You'll find that all 50s have their detriments and weaknesses so you either have to settle or choose a selection of lenses so you can cover your wants. I find that the Sigmalux is sharper than the ZE 1.4 wide open through 2.8 and may even have better contrast too, especially when near mfd.I think you'll find that the 50L won't be acceptably sharp until past f2 and still won't compare to sharpness of the Zeiss 50MP. The 50MP does sound like the best choice as it seems you primarily want sharpness or at least that's why you so readily dismiss other lenses.
on a Sony NEX or any m4/3rds camera
