with A9, you can autofocus using all points up to F11, when using extenders.
Sounds nice, but I just realized that if we're shooting continuously while the subject is moving toward/away from us, we are forcing the AF to do so at the shooting aperture (let us assume f/11, for discussion) and AF precision is affected 'to within one DOF zone (at f/11)', so AF is not tracking so hot at that aperture, is it?!
At f/11, 800mm has a 20/20 vision 6.9' DOF zone at 250'. At 500' distance we're framing 15' x 22.5' and our 20/20 vision DOF with 800mm at f/11 is 28'.
Is AF at that aperture of much use with fast moving subjects?