The D850 is the D500 and a D850. The 850 crops at 20 mps which is the same as the D500. There is NO advantage using the D850 in crop mode, just the opposite. When you put it in crop mode you are narrowing you DOF (more difficult to shoot BIF) and very often I get the opportunity when shooting birds to not crop and take advantage of the whole banana (45 megapixels). In crop mode the D850 GIVES YOU NO EXTRA REACH! You do the same thing in PP that is narrowing your depth of field and cropping comes in at 20 mps. I've had the Sigma 600C, rented the nikkor 500, and now have upgraded to the Tamron 600 G2. The Tammy gets consistently sharper images than the others but some will disagree. Most have never shot birds/BIFs. (you're sure as heck aren't going to be using a tripod unless you're at the bird feeder)

If money is no object, the 600mmF4 FL is the way to go. You are not going to find a sharper lens.

If you are looking for value, look no further than the Sigma 150-600 S or the Tamron 150-600 G2 - these are built quite well, have weather sealing, and cost $1800 and $1200 respectively.

You can get a 200-500 Nikkor, which is still a very good lens, but if you want longer than 500mm, you'd have to use a 1.4xTC and there goes the image quality and the reason you purchased a D850.

I had an older 600mmF4 which was my main lens for shooting birds. It was heavy, which meant the tripod and head was a constant companion. I started using a Sigma Sport in 2017 and found it every bit as sharp as my 600mmF4, a little more than half the weight, which meant I could use it hand-held, and much easier to grab and go with. When shooting it is attached to my Black Rapid sling strap by it's tripod socket.

FWIW, I borrowed the 200-500 from NPS, and found it sharp and crisp, but I found the Sigma to be closer in quality to my 600mmF4 than the Nikkor, and I enjoyed the extra 100mm. The fact the Nikkor is not sealed against the envirnment was a non-starter for me. It is not unusual for me to go out in light drizzle, to the seashore for shore birds, and other environments that are inhospitable to an unsealed lens.

I've since shot a bit with a friend's Tamron G2 and found it every bit as good as the Sigma and virtually indistinguishable from shots taken with the 600mm F4.

This was taken this past August with the Sigma Sport at 600mm, F7.1 1/800 sec, NIkon D810.

I have a D850 and would like opinions on what combination of lens/teleconverter/image area I should use to get the best result. My longest lens is a Nikkor 80-200mm f/2.8. Today I took some test shots of a street lamp outside the house, first with the lens only @ 200mm using the FX image area. Then I shot the same scene using the DX crop image area. And finally I shot it again using DX crop and a TC-200 teleconverter. I cropped the resultant images so that the lamp was the same size and was surprised that I couldn't pick between any of the images. Or is that what I should have expected? Should I have shot using the FX image area and the teleconverter to improve things?