I noticed my Canon Mount Sigma 50 1.4 has a serious case of AF inaccuracy only when AF Assist is fired? Is it a common problem and is it rectifiable? Its not a huge problem since its a low light lens and can AF usually without assist but just want to know if any one else is having that problem and if it can fixed by Sigma Service. Also at infinity is seems to have a low af hit rate (50%). Do people recommend to send in calibration for it?