Like some other people have said, I had trouble nailing focus with this lens at 2.8. The 5dIII focus screen was just not accurate enough to portray fine focus in such a wide FOV
It's even quite difficult with the precision matte in my 5DII. The solution is to get an AF confirmation chip and calibrate it to give you confirmation at the aperture you desire. Be aware there is some focus shift in this lens depending on the aperture used! I use the precision matte to dial in the focus at f/2.8 to f/3.2, and have the AF chip calibrated for f/4 which gives reliable results from f/3.2 and up.