I had an earlier Sigma 50 1.4 that I did not use much. When I started using it I found it awful so I sent it back to Sigma for service as I was convinced that it was out of spec.
Sigma reported that an element had come loose and they had re-assembed it and it now performs beautifully.
Unfortunately, I find I seldom use this FL so the new one is of no interest to me. However the 35 does interest me.