Sigma recently announced that the use of its Mount Converter MC-11 and certain Sigma zoom lenses with the Sony Alpha a9 (Amazon/B&H Photo/Adorama) may result in missed focus. According to Sigma USA, the issue may arise ‘occasionally’ when using AF-S mode and while shooting ‘in the telephoto range.’
Currently, Sigma is working on this issue, and Sigma will soon announce a firmware update MC-11 mount adapter. Stay tuned for more info.
From Sigma:
We have found that the combination of SIGMA MOUNT CONVERTER MC-11 (SA-E and EF-E) and some of SIGMA’s interchangeable zoom lenses may not focus precisely, if it is attached to SONY a9 (ILCE-9), released by SONY in May in 2017.
When using AF-S mode and shooting in telephoto range, it could occasionally display the shot is in focus, although it may not be spotted properly on the subject.We are afraid of this inconvenience caused to the customers who use these products.
We are now investigating the phenomenon and will make a further announcement on the website shortly.