Since switching all my camera bodies to full frame last year, I’ve been using Canon’s 2x mkIII teleconverters much more frequently now. In the past the mkII version wasn’t the greatest as far as sharpness was concerned, but with the improvements made to the newer version, I’ve been nothing but impressed.
I frequently pair it with my 500mm and 300mm lenses and sharpness, contrast, and speed have not only been acceptable, but very good.
For this shot of the pronghorns taken in central Montana, I paired it with my 500mm giving me a 1000mm to help fill the frame. I rarely leave home these days without this converter being in my bag.