Thanks for picking up on this. The 500mm was checked out/calibrated etc about a year ago by Nikon & has worked splendidly on my D3s with the 1.4X & 1.7X as I had posted. I did contact Nikon & they were less than helpful re. the D600. They recommended sending the body, the 1.4X & the lens to them as a repair. I'm reluctant to do that since the shipping cost will be off the scales. If there is someone that has the lens & D7100 & has had no issues with the 1.4 or 1.7 then I would absolutely agree it has to be the lens. So a big thank you passing this thread to a fellow Nikonian!