Announced in early 2011, the Canon EF500mm f/4L IS II USM and EF600mm f/4L IS II USM are updates to two highly respected lenses aimed squarely at professional wildlife and sports photographers, or those who want the best image quality possible at these focal lengths. These Mark II versions aim to take everything that was good about their predecessors and turn them up to the max.
Further readings for the Canon EF500mm f/4L IS II USM and EF600mm f/4L IS II USM review – Giants in the range
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It's great to see you guys take advantage of your new lab to test the big lenses and it seems the Mark II lenses are worth their high prices. I don't know how big a focal length you can test, but is there any chance you might do a test of how teleconverters affect their performance? From Canon's MTF charts and some of the tests I've seen online it looks like these lenses are still amazing, even with the 2x extender.