For me personally I find the 50L unacceptably soft. If you can see a clear improvement in sharpness in 1024 px in uncropped frames like I can then there is a huge difference. I really don't know what else to say. You can also make the sigma images look just like the Canons @f/1.4 in the in focus areas, just by softening the sigma up, literally there are light room settings that make both indistinguishable for in focus areas but you can't create detail that was never recorded with the Canon. Logically speaking if you get a good copy and ignore the very minor difference between f/1.2 and f/1.4 the Sigma is better in every measurable and conceivable factor. I really don't get why this is so contentious.Yep, no doubt there is a big difference if it's visible in 1024px photos. The Sigma wins for sharpness at f/1.4 and for richer color, and for that it's going to attract a lot of photographers. The difference is certainly visible in the full-res uploaded to Dropbox (Thank you Jason for that).
However, while sharpness and detail are important, they aren't everything there is to know about a lens. The 50L adds a beauty factor in how it draws a picture. It just looks pretty, especially for portraits. Maybe the 50L's slight softness is an advantage for some photos and for some photographers. I'm eager to see some portrait comparisons with the Sigma 50/1.4. I'm sure the Sigma will compare well, but it will be interesting to see any differences.
So does that mean that you don't want the 50L to be sharper than the Sigma 50 Art?
If sharpness is your only concern, you shouldn't buy either of these lenses and instead get a 50mm f/1.8 and shoot at f/8, which I'll be much sharper than both of these at 1.4.
sigma is a little sharper at f/1.4 but can't do f/1.2 at all, can't get quite as thin DOF as the 50L, has less realistic color and less pleasing boke. Personally no, I would not trade off those things for a.little more sharpness; I have a ton of sharp lenses but very few look quite like the 50l.
Ah, well if bokeh becomes a factor, there is no doubt that the 50L offers thinner depth of field and hence can render more out of focus. This is not subjective, but rather a fact of f/1.2 vs f/1.4. I also like the rendering better, though that is subjective. But that is the whole point, one can crow about sharpness all they like but that is a very one dimensional and frankly inaccurate summation of a lens.
Well...I think the whole point is sharpness (wide open) and nice bokeh. So many wide lenses are not even close to sharp wide open... I agree tho. I think that both of these lenses (and the Sigma 50mm f/1.4 DG) all have good sharpness wide open and they all have different renderings. I can totally see where someone would like the Canon, as it is clearly no slouch and does have the superior softness in the rendering of the bokeh, but I also feel that the sharpness and contrast is superior in the Sigma (and the price) so I can see it is personal like or dislike for each look. (and I am not even going to touch the AF discussion...it is so complicated...arrrrgggghhh..LOL).
I think all three of the lenses I am mentioning are great for different reasons. Makes buying just one kind of tough...but then if you have a REALLY good shot...is anyone going to notice which lens it was??? LOL. That is a whole other discussion too!!!!