Sigma Corp issued this official statement today regarding an upcoming firmware fix to allow Sigma MC-11 mount converters (available from B&H Photo) to work properly on Sony a9:
For customers using Sony α9 (ILCE-9) Thank you very much for your continued support of Sigma products.
This time, in the combination of our MOUNT CONVERTER MC – 11 (SA – E, EF – E) and some zoom lenses to α9 (ILCE – 9) which was released from Sony Corporation on May 26, It turned out that the phenomenon that the in-focus mark lights up occurs even if the subject is rarely in focus when taking pictures at the telephoto side with AF-S.
We deeply apologize for any inconvenience to our favorite customers.
We are currently conducting investigations. If the cause is found out, we will inform you again on our website.
We sincerely look forward to your company and our products.
Sigma Customer Support Department, Inc.