My impression is quite good regarding the Sigma 35 and the combination with the 5d3.
Focussing is accurate in all the shots I have taken so far. Which I can not say of the 50 1.4 Sigma, which has a bigger rate of unsharp photo's.
The 85mm 1.4 from Sigma is very precise as well regarding focussing, so it seems the 35mm might have the same bit of software/hardware for focussing as the 85mm.
So far there are a bunch of things that I really noticed with the 35mm from Sigma:
- It's very sharp, from 1.4 onward. No need to stop down for sharpness.
- Build quality is very good, better then anything I know from Sigma and on par with Canon 35mm 1.4.
- Even the front and back cap are new and have a better construction
- Lens does not need AFMA and is sharp straight out of box
- Sigma colors are present in this lens as well, so a bit of a yellowcast. The 35L has more red/blue coloring and gave the photo's more spark/vivid/positive feel to it. The yellowcast from the Sigma is not perse my favorite. Though it bothers me little and goes away in postprocessing in a flash
- Focussingring is nicely damped and smooth, but has to much friction to be operated easily with one finger. The 35L has an easier focusring to turn and can be used with one finger (as I always like to do).
- Focussing speed is about the same as the 50 1.4 and 85 1.4 from Sigma that I also have. The 35L is a wee bit faster focussing, but not by much. Next to eachother, you notice the difference, but they are both fast enough for wedding/action/walking/sprinting.
So far I really like the 35 1.4, what I notice most of all is the sharpness at 1.4 already and that is unseen with the 1.4 Canon 35mm.
If they were both exactly equal pricing, the Canon and the Sigma, I might go with the Canon, due to the colors.
But since there is a 400 euro difference, I believe I have made the right choice to go with the Sigma tool.