Automatic extension tubes create sometimes vignetting while used with fast wideopen tele lenses - one can solve this issue
I reconized vignetting while using an EF mount converted Canon FD 300mm f/2.8L on my EOS 5D with extension tubes - at least with wide open aperture.
I even had the same with a Novoflex 400 mm f/5.6. Because of this problem I decide to modify the Kenko extension tube.
The tools for this modification are pretty simple: Some files. And probably screwdrivers when you want to file it disassembled - but I prefered pressured air for cleaning.
It is even not very time consuming, probably 30 minutes per extension-tube.
Just file the too small diameter to a ~ rectangular shape more like your sensor looks like
- and in the right direction to fit the passing light rays to your camera sensor or film orienation.
Please take care to clean the extension tube afterwards! Dust or chips inside the lens or camera could cause defects!