I used one of my D810 Nikon's with 300 f2.8 AF-S lens
Plus the 20EII and 1.7E extenders. Effective 1020mm f10
Note that the camera does no recognize the added 1.7 in exif,
however it will expose correctly. Focus hunts and is very poor on this body.
Using the D810 stacked with 20EII and 14EII, (840mm f8) focus is swift and solid.