Tigger1 Loc: Surrey, BC Canada

Rent both simultaneously and test them both in the same light on the same subjects, then you may have a resonably acccurate test that records what you would use either (or both) lenses for in the real world. Only you can make the decision which is best for you.
For example: do you always shoot handheld or always tripod mounted? I have both lenses for use on my D800, D750 and/or D7200. I find both lenses are great for various wildlife/birding uses. The truth is that I rarely use either lens since I bought into the Olympus micro 4/3 world and added lenses in micro 4/3 format that duplicates the focal lengths covered in the FF and APS-C formats.
Good luck with your choice.

via the lens Loc: Northern California, near Yosemite NP

I've got both. Once I got the 200-500 I hardly ever used the 80-400 again. My recommendation is the 200-500, or, better yet, the Nikon 500mm PF, which you can easily hand hold...as soon as I got the PF I've hardly used the 200-500, although on occasion I do need the shorter range.

amfoto1 Loc: San Jose, Calif. USA

Since you are using full frame (FX) cameras, you need all the focal length you can get to shoot wildlife... particularly birds. Based on that.... at LEAST get the 200-500mm. But even longer like a Tamron or Sigma 150-600mm might be an even better choice.

Between the Nikkor 80-400mm and 200-500mm lenses, the 80-400mm is the sharper. But it will come up short for birds and other small wildlife, on an FX camera. The additional 100mm of the 200-500mm would help, but still will come up short a lot of the time on FX, for smaller critters.

I notice that the Tamron 150-600m "G2" is on sale right now, too. On this lens and some other recent Tamron telephotos, they've thoughtfully incorporated an Arca-Swiss dovetail into the tripod mounting ring foot, too. That will be a nice feature to have if you eventually want to put the lens on gimbal. I wish all lens manufacturers would do that! (You'd need to buy a lens plate or a replacement tripod ring for either of the Nikkors.)

Another approach would be to get a DX camera to use with one of the Nikkor telephoto zooms. Maybe trade in the D750 for a D7200, if you can find one. That would give you 24MP with a "free 1.5X teleconverter"... Where by "free" I mean there's no loss of light like there is to an actual teleconverter. Where a 200-500mm f/5.6 with an actual 1.5X teleconverter would become a 300-750mm f/8 combo... On a D7200 it would "act like" a 300-750mm f/5.6. No teleconverter needed.

You also could put your D850 into DX mode when using the longer lens... but that would make for approx. 19MP images, versus the 24MP possible with a D7200.

Besides that, there's no difference what-so-ever simply cropping the D850 image in post processing instead, rather than using the camera in DX mode.

RickL Loc: Vail, Az

I have a D850 and use a Tamron 150-600 g2. It gives me good reach for wildlife and is tack sharp at all focal lengths. My wife has a D7200 and uses her own Tamron 150-600 g2 lens. Make sure you only get the generation 2 lens. It is an extraordinary lens and provides plenty of reach.

JeffDavidson Loc: Originally Detroit Now Los Angeles

The 200-500mm works fine on an FX

The 1.5 factor can be achieved on an FX by using a smaller section in the view finder and enlarging it. Then it is just as good if not better than a DX.

You will be happier with the longer lens for wildlife and birds. You may not need the 1.4 teleconverter but it can be used on the DX or FX

tca2267 Loc: Florida

I have never did a fine tune adjustment on a Nikon lens...
as Nikon has told me that their
lenses come right  out of the box "tack sharp"....
Was the fine focus tuning easy to do?

THRYLLOS

I own both lenses for some years and I had pre ordered the 200-500 when it first came out. Both lenses are good and, for me at least, have different functions.
The 200-500 is cheaper and has constant max aperture at f/5.6 whereas the 80-400 has variable aperture and I am not a fan of that.
The 80-400 is smaller and lighter and takes 77mm filters; however, when I put a B&W polarizing filter on it, it would not focus with my D800. Took the lens to the repair store and as soon the guy unscrewed the polarizer, the lens focused fine. The 200-500 takes 95mm filters and the polarizer works well there.
Usually, I take the 80-400 when I travel by airplane because it is smaller and lighter and the 200-500 when I drive.
Valuewise, the 200-500 is a better option as it is cheaper and has more reach since you will need maximum reach for wildlife, especially birds. With a 1.4 teleconverter it becomes 700mm and that will be enough in most situations.
Go for the the 200-500mm and you will not be sorry for this.

tienws

I have both. And both serve me well. It bogs down to the environment you are shooting. For example, Africa Safari is vast and most animals are far away. So the 200-500 is better. For Ecuador Galapagos, animals are both close and far, so the 80-400 is better suited. If you can afford both, put one on each camera.
Or for a lighter weight, I use the 70-300 PF lens on one of the camera.
Since getting the 70-300, I am using it more and more and the 80-400 stays home as I always go out with 2 cameras.

tca2267 Loc: Florida

Thanks......

Tell me about the 70-300 PF..........is that Nikon lens....

tienws

Yes, it is a Nikon lens. I have the FX version, not the DX version. Light and cheap too.

CO

Isn't the 70-300mm an AF-P lens? The PF lenses are the phase fresnel lenses. They are the 300mm f/4 PF and 500mm f/5.6 PF lenses.

tienws

AF-P lenses use a “Pulse” motor or “Stepping” autofocus motor and are even quieter and smoother to autofocus than an AF-S lens.
Like you said, PF lenses are the phase fresnel lenses.
They are two different technologies applied to 2 different areas of a lens design.
Hope that will help.

usnret Loc: Woodhull Il

Rent them both and you will be able to answer your own question. You can put each to the test by focusing on fixed objects (doesn't have to be critters), at various distances. It helps take the shoulda, coulda wish I woulda out of the equasion.

TonyBrown

I used a professional camera lens calibration company. All of my Nikon lenses needed calibration even the primes.

Trogs

I
I had the same dilemma but finally decided on the Tamron 150-600 G2. It has the reach, it is not as heavy as the Sigma Sport so better for handholding. It has a dedicated VC for panning and it is very well made with good weather sealing throughout the lens. I use mine coupled to a Nikon 7500. Finally I believe all the lenses mentioned will produce great shots!