I am not sure this is a big deal for some of you guys, but i was never able to get a sharp flight shot with the MK III with the 2xtc attached to my 600 or 500mm. This BCNH flew by and i was able to acquire and maintain focus for a 10 shots burst while he was flying against a busy Bg and blue sky.
MK IV
500mm x 2tc
F 10
1/2000
Iso 800