Sigma 50 f1.4 soft?
Post a pic and we'll look at the data and then comment, I'm betting something will show in the data. The great majority of times its user error. If you are using the 50mm on a ASP-C sensor camera - for example, Canon - then the lowest you can go with the shutter speed is 1/80; on full frame 1/50 sec.
We need to see the pic and data.
Hindsight is better than foresight, except for lost opportunity costs.