Hi all,

A friend & I are trying to make an informed decision and I would love to hear from some of my "trusted Hogs" to assist.

OK - here goes, we both currently own/use/love our Nikon D500's (crop sensor), and are considering adding a FF NIKON with the intention of improving low light performance at higher ISO's over our D500's. The ONLY models under consideration are the D850, D5, D6 , and Z9. The main reason for wanting to add a new camera is for better performance in low light. Our main subjects are birds, BIF, and wildlife. I'm fully aware of the limitations of mirrorless when it comes to what I shoot, as well as the specs on the cameras under consideration. The ONLY thing I'm asking about is low light performance. All our lenses are FX.

1) Which of the above cameras gives the best performance in low light at higher ISO's, and why? D5 & D6 (both FF) are significantly lower MP than the FF D850 and the Z9. Feel free to get technical!
2) If I were to use the D850 or the Z9 (both much higher MP than the D500, D5, and D6) in one of the crop modes, wouldn't the pixel DENSITY remain the same vs. the FX mode on that camera thus making NO difference in low light performance? If my understanding of pixel size/pixel density is correct, I would think that the low light performance of the D850 or the Z9 would remain the same FX mode vs Crop Mode in each camera.
3) Given my understanding, noted in 2) above, wouldn't the low light performance of the D5, D6 be better than that of the D850 or Z9 using one of the available crop modes? (I know that the D500 is a crop sensor, and therefore has higher pixel density than either the D5 or D6 that have about the same MP count as the D500, and therefore the D500 would NOT be as good in low light as the D5 or D6.)

I realize that the processors in the cameras under consideration differ, and that those differences may come into play when it comes to low light performance. Feel free to mention those differences if they make a difference. I'm not technically savvy enough to know the differences between processors.

After reading through this, it seems that my question has come down to the crop sensor D5 & D6 vs the FF D850 vs Z9.

Please feel free to correct any of my misunderstanding(s) of this subject.
Thanks in advance!