Based on the DPreview report I bought my first Sigma lens, the 50 mm f1.4. I have just finished evaluating it properly to make sure I have a good one. I compared it to the Canon 50 mm f1.8 ll & my 17-40 L f4.0. I have a Canon 20D currently. I used raw files in CS3 for my evaluations.
There is no point in buying a lens with a large aperture if it's useless for taking photos at full aperture, sounds obvious perhaps but so many lenses are made like that! I am very happy to say this Sigma is not one of them!
At f1.4 it is sharp in the middle, only the corners are a little soft but very similar to my 17-40L at 40 mm at f8! ( I am not happy with my wide L zoom for large prints).
So the really good news is you can actually use it at f1.4! The depth of field at f1.4 renders the edges of little concern. As you stop down the sharp zone spreads out, by f5.6 the edges are very good, by f8 as good as the centre. I see no change between f8 & f11 at all. The Canon f1.8 ll is fairly sharp in the very middle at f1.8 but dreamy everywhere else.
As you stop down the Sigma it gets a little sharper in the middle but by f2.0 any change is very small indeed. At f8 it is very sharp corner to corner. My Canon f1.8 ll is also as sharp at f8, I can not see a difference in sharpness, however there are other differences.
Shooting into the light with the sun in the corner of the frame has NO flare at all, the tree branches have no chromatic aberrations whatsoever (even at 200%). My 16*24 print is perfect! (also no vignetting!).
The Sigma has a far better color rendition than the Canon, also the images have an almost 3D quality to them, relaxing to look at.
One strange thing I noticed; if the sun is just out of the field the picture is flat, lacking contrast (with lens hood). It may be an exposure issue I will have to investigate when I have time.
Unlike the Canon f1.8 & f1.4 for that matter the Sigma is usable over the whole range of apertures, it does exactly what is was designed to do. The focus speed is also fine.
I bought mine at Vistek, with the weak dollar it hurts but that may get worse as the Yen gets stronger! At least in Canada we get a 10 year warranty.
The Sigma build quality is excellent & I love the locking lens hood, very secure. It comes with a real case offering serious protection, which is good as my bag is full!
I bought it for portraits, some landscape & commercial work, all my other lenses are Canon. I have gone off wide zooms for large prints & from now on all my short lenses will be primes.
After using it for a week I discovered my Canon battery is drained even when the camera is off, not rapidly but enough to give a dead battery when you did not expect it.