I had an old vivitar autofocus 2X Tele converter in film days. As the digital era started it turned out that these converters can generate worst kind of image, so it went out of service. But why not through away the inner glass element of this converter and turn it into an extension tube. So I removed all the glass component and covered the shiny aspects with black felt found in the opening of good old film canister. Now I have a fully functional AF Extension Tube. Which works perfectly with my Nikons. And it's solid too. I have been using it for a long time and it has become a permanent part of my macro setup.