I am using this new lens with both the D800 & D7100. If in AF-S mode, focus will be achieved very quickly at all focal lengths. If in AF-C or AF-A, focus can be achieved until 300 and above. Once I hit 300, the lens does not "hunt" but the range finder in the viewer of both cameras shows focus is not achieved because the circle and arrows alternate.. I have no problems using the 70-200 with 2x extender handheld. Focus is very quick and locks so I don't think my technique is too far off. I am not able to test on a tripod at this time. Any thoughts appreciated. Would you send this lens back for a replacement?