I use back button focus and on the D500 and it just doesn't seem to make a mistake in focusing. However occasionally I have missed a few images on getting them tack sharp. Well i figured out the problem, in my excitement to press the AF ON button on the rear of the camera I mistakenly put my thumb on the joy stick instead. This is a button that I am not use to being there so I need to pay a little more attention when I am shooting. Thought I would post this just in case anyone else might be having the same problem and not realize what was going on. There is nothing wrong with the camera, I just need to get accustom to the button being there.