I was shooting dragon flies last weekend with my Nikon D810, Nikon 300mm f2.8 G VR II lens and an Kenko extension tube with electronic contacts so AF would work. I was surprised to see that there was vignetting in all four corners when the f stop was at f 5.6. Vignetting was less noticeable at f4 and f2.8. I did not have my Nikon set of tubes with me. These gave a wider diameter so you do not get vignetting. Does anybody know of a brand of tubes with electronic contacts with a wider diameter than my Kenko set so this problem would not occur again? My Kenko set is about 10-15 years old.