I agree. I meant to say when 750 mm DX is used, one can fill the frame, therefore useful for AF & bird eye AF especially for smaller birds. Often, small bird eye AF doesn't happen on the 500 mm when it is tiny in the frame.
The focal length usage depends on one's style of photography.
I try to fill the subject, mammal especially only between 8-20% of the entire frame to show its habitat.
In such cases, 560/500 mm to 400 mm makes a considerable difference especially when one in on a safari in tropical rain forests with limited scope to move backwards.
100-400's f5.6 on the long end is not too appealing. Also, it is apparently not as sharp as the 400 f4.5. on the long end but nearly as expensive. IDK.
Either way, what are the major practical advantages that you have been experiencing using the S line telephoto glass?
Are you on Flickr or Instagram. I would be happy to see the pictures.
I am considering buying a 400 f4.5 after selling my 500 f5.6...
While I am replying to this "response," my comment will be more general and relate to an obsession with "new" or "best" rather than "optimal for the use case".
To me, this is one of those "popcorn" threads where one has to watch where the discussion goes. I recognize a lot of photographers in this discussion from FM (on which I am "owlseyes"), and many of us have abandoned outstanding lenses in order to gobble up the latest and greatest with an insatiable lust. In the end, we obsess about optimal optical quality, when this slightest of advantage is rarely visible to anyone but another pixel-peeping photographer.
Sid in your response you state "100-400's f5.6 on the long end is not too appealing. Also, it is apparently not as sharp as the 400 f4.5. on the long end but nearly as expensive. IDK." ...
In response, I offer the following... (1) 400mm f4.5 is less than a stop faster and most (including you, I believe) will be adding a 1.4x converter making this a slower 560mm f6.3 lens that will be even slower than the 500 f5.6 you currently use. Now I certainly understand the value of f4.5 if 400mm is the focal length you want to use, but many people discussing this lens are looking for a way to replace their more than adequate f5.6 500mm lens. To be clear, there are many reasons to choose the Z-mount lens... top on the list is the ability to do away with an FTZ and "cross-compatibility."
(2) With respect to price and sharpness in the 100-400S v the 400mm f4.5, it makes sense that a $3250 prime (US) lens is going to be sharper at its ONE focal length than a $2650 (US) zoom lens designed to be used across multiple focal lengths. Both lenses represent a compromise here... sharpest optical quality vs focal length flexibility. Plenty of reviews suggest the 400mm f4.5 is sharper (dare I say "duh," but... nobody says the 100-400 is not sharp throughout the range... see Ricci's discussion). As one who has been on safaris, shoots from canoes / kayaks / zodiacs, I have found zooms to be the best choice under these conditions.... YVMV.
But back to my first point... the obsession with newest and best. The latter almost never results in better pictures. In Sony's world, they have a 100-400 and 200-600. The 100-400 is sharper, faster, and lighter than the less expensive 200-600, but most people pull out their 200-600 first because it extends to 600mm. Most are willing to sacrifice the loss in AF speed, aperture, and brute optical quality in order to eek out a bit more focal length. The same will probably be true if Nikon ever produces their unicorn 200-600mm lens. It too will be slower, less sharp, and heavier, than the 100-400 (or 400 f4.5)... but it will be the right lens for many many wildlife photographers.
Right now the 400mm f4.5 is the "fashionable" less expensive super-tele... it is a great idea, it belongs in the Nikon line-up, and it is worth owning... but is it the right lens if you need to shoot at 500mm (+) or require flexibility? Only you can answer these questions.
And now for my final point... I often fall for the "newer is better" mindset. When I get bored and do not shoot enough, I begin thinking that the problem is with my lenses, my cameras, my computer, etc... Retail therapy is the placebo, but it is not the cure. Point in fact... I purchased the 100-400S to replace my 200-400VR (newer is better, right?). With the exception of some work in photo blinds, this new (and described as amazing by many) lens has not seen the light of day. It is a fantastic optic, easily beats my former 200-400VR in quality and weight, but it adds very little to the way I see the world. Some might say it's a step back, but I'm considering the sale of my 100-400 (not because of some perceived inadequacy) to buy a 300mm f2.8VR. The latter has become so cheap relative to Z lenses that it drifts into bargain status. It is different than the Z optics, it is a fast lens that produces a unique bokeh that has the potential to add something very different ...
While I realize I have become long in the tooth... I think it is important to stop and contemplate the real motivation behind the decisions we make.
cheers,
bruce