Thank you, Linh! The sigma lens works fine with A9, its AF is fast enough for most BIF shots for me, although not as fast as the native lens FE100-400mm. I just got the Sigma 1.4x TC1401, it does not slow AF much with latest Sigma firmware update for MC11 (faster than with Canon 1.4x III that I used to use with this lens). Here is a shot today with the Sigma 1.4xTC. (Edit: forgot to mention that Sony A9 is an amazing camera, it shines with any lens I throw at it )
That's an incredible shot, Shalu, especially with a teleconverter!