wcuster wrote:
You wrote: "Most suggest you do not use above a 1.4 because your camera can only focus down to f8." ??????
If it is an f/4 600mm lens with a D810 it will autofocus with a 2x extender. If the 600 mm lens is not f/4, i.e., has a smaller max aperture (f5.6 or f6.3) then a lesser (1.7 or 1.4) extender would bee needed.
Yes, that amplifies the point.