1D X Firmware Update provides f/8 AF and AI Servo AF point illumination

[Sean Setters]

See Bryan's post:

http://www.the-digital-picture.com/N...aspx?News=3227

[Dr Croubie]

FWIW, 7D + Kenko 1.4x Pro300 DG + 70-300L focuses fine in good light (albeit, a bit jittery and has trouble locking sometimes).
So I've no doubt that a Canon-brand could AF on the 1DX in theory (although as pointed out, the rear element does protrude a long way out both at close-focus and 70mm-end. There's no guarantee that it won't try to hunt to close-focus, I wouldn't be trusting $2k worth of glass with it, i'm sticking with my Kenko)