400mm Telephoto lens f5.6 on Canon EOS APS-C Camera 640mm (35 mm equivalent focal length) EF 200mm f2.8 L II USM with Canon Extender EF 2x III AF speed test sample images
No problem with focus speed and acuracy, image quality on full frame like 6D is superb, but I donˇt recommend this combination to use with APS-C, 2 time extender is simply too much for.
