Be warned, there is something addictive about the Z 400mm f/4.5 VR S.
It is an absolute pleasure due to the super low weight and the excellent weight balance for such a lens.
Even with the added weight of a Z TC 1.4x, you save a whopping 800 grams compared to a Z 180-600.
Using it at 400mm f/4.5, it's already the weight of a Z8 body it saves, around 1 kg and it is in addition significant more compact.
For me there is no more turning back, flyweight for the win.
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Small appetizer, including some surfing action.
Other advantages of the Z 400mm f/4.5 is the speed of focus (much faster than either the 100-400mm or 180-600mm) as well as the ability to use the Z 2.0TC. But if one needs 300mm, then the 400mm f/4.5 is not the lens for you.
We really need a 200-500mm f/4 Nikon. Even variable 4.5-5.6 would be killer
I’m picking up a 400mm f/4.5 tomorrow to play with regardless.
“Much faster” is an arbitrary term; it may be
faster but much faster vs. the 100-400mm doesn’t necessarily ring true. I’ve begrudgingly watched all of the YouTube nonsense reviews with the color checker boys and their tests.
And faster for what? Birds in flight? F1? Air show? All depends on the subject matter.
Sure, it might be 1/4 of a second faster but doubtful real world cases will experience these differences. The 180-600mm OTOH is slower than both, but again, depends what you’re shooting. Totally fine for some or most.
It’s like the various YouTube reviewers that sit around and take pictures of color checker cards to determine what’s a “better” lens - absolute rubbish all these reviews. I cannot understand why anyone (including myself) watches this crap.
It’s always about
real world use, not taking pictures of color checker cards and quantifying that one lens is “sharper” than another because the line of the letter “T” is 1 pixel sharper as viewed by someone else’s eyes and post processing work on a YouTube uploaded video.
A great lens takes into account a ton of factors: quality of the glass, coatings, backlit subject performance, CA, vignetting, corners, speed, you name it.
Nobody can sit here and tell me a $3k lens (the 100-400mm) is better than a $1600 lens (180-600mm) if we solely look at price

erformance. Same goes for the 400mm f/2.8 - it smokes every lens in the lineup. And not only because it’s fast but for a variety of other reasons. Photography and optics aren’t only about sharpness and color checkers
End rant here but the YouTube reviews with the color checker nonsense and half the forum saying “OMG the 180-600mm” is the best lens Nikon’s made ever is just amateur talk. Need to stop watching this crap.
I’ll give it to Thom Hogan though, his reviews are always complete and take into account all of the important factors that go into a great body or lens.