Tony Northrup recently has posted a video comparison on Canon EF 100-400mm f/4.5-5,6L IS USM lens and Canon EF 70-200mm f/2.8L IS II USM lens w/ Extender EF 2X III.
From Northrup:
The EF 70-200mm f/2.8L IS II USM lens w/ Extender EF 2X III has same image quality as EF 100-400mm f/4.5-5.6L IS USM lens. See the video comparison above.
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