All Sigma lenses have focusing problems! I had two of the a 24-700 and 70-200 last versions - I sent them to be calibrated with my body and then everything was ok.
This is FALSE. I have two Sigma lenses that have had no focusing problems. (I don't consider improving AF accuracy with in-camera AFMA to be part of a "problem.")
But the 50 Art that I received did have erratic, unreliable AF that could not be helped with AFMA.
So, if online chatter can lead to Sigma fixing the problem, we aren't helping by using sweeping, ranting statements such as "All Sigma lenses have focusing problems!"
I stated this before and say this again: I did not and do not have AF problems with my 50Art. And I agree with YuengLinger. It is wrong to make statements like "All Sigma lenses have focusing problems!" - it is not true. Sigma did a great job with the Art series.
My second copy of the Sigma 50/1.4 (non Art) focuses very accurately and consistently. The same can't be said of the first though