Back from a "test and tune" session with the 500 II, where I did some MFA with my 5D4 and 80D, and the 1.4/2.0 extenders. Nothing needed anymore than +/-3 points of MFA, which is pretty nice!
Here's some initial comparisons with the 400 DO II:
The 500 f/4 II is noticeably more bulky. After an hour or two of messing about, it gets heavier for sure, but what fatigues me the most is the girth. I have to support the thicker lens with a more open hand compared to the skinny-mini DO II. I'm sure I'd grow to accustomed to this after a while, and I need to get back in the gym anyway.
Maneuverability: not as swingable as the DO II. This thing requires a deliberate action to get it on target. Once on target, the IS is pretty good with assisting, but keeping target under AF point is a little more difficult than the DO II. With good support though, not a problem.
AF speed (bare lens vs. bare lens): almost a wash, though I have to give a slight, slight edge to the DO. Not enough to make a difference in anything I do.
1.4xIII: very quick for both lenses, but I don't think I can make a full determination as yet.
2.0xIII: expectedly slow on both lenses, any interference from things like twigs, all bets are off. On the 80D, you get one center point, and it's dicey. 5D4 is better with this TC.
surprisingly, IQ-wise I don't feel that the 400 DO and 5D4 make as good of a combo as the 400 DO + 80D. With the 500 II though, the 5D4 demonstrates that same crispy, detailed look that the DO II can produce on my 80D. I'm not anywhere near ready to proclaim which lens is better, but for now I'm just blown away by the IQ the 5D4+500 is capable of:
80D: just as good here as with the DO II. I need to get more time with this combo before making a final determination, but I think I am good to go:
EDIT: the 80D works well with the 500:
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Photo from MatthewK's gallery.