from Tom Hogan:
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Compared to the 100-400mm f/4.5-5.6 VR S at 400mm f/5.6, the 400mm f/4.5 VR S is somewhat better wide open, pretty much across the board, and clearly better at the same f/5.6 aperture.
Finally, the 400mm f/4.5 VR S is a little better with a 1.4x teleconverter than the 100-400mm f/4.5-5.6 VR S, but both are quite good. In the F-mount world I was not a fan (or user) of teleconverters. In the Z-mount world, I'm coming around to liking (and using) them, at least the 1.4x version.
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Note that the zoom is 60g heavier but has a much better close focus.