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| p.4 #9 · Canon EF 200-400mm f/4L IS 1.4X: Quick Comparison |
OK, then that would be 1200mm > 784mm nominally, assuming that 200-400L does go to 400mm in reality, as there have been some suggestions that it might not be quite that long.
Either way, the lens should be capable of nominal 800mm f/8, if 2xTC is mounted on the "bare" lens, i.e., with the built in TC out of the way. Ron Scheffler might have done some shots with that combination.
Some folks have gone to 1120mm f/11 with it, but that is obviously MF stuff. See this link.