"Buy/use a D850 when your most of the time shooting in reasonable light."
"Buy/use a D5-D6 or the like when you encounter more challenging lighting conditions and use the FLs to compensate for the lesser rez."
Sorry to sound ignorant, but I don't know what you meant by FLs?
Thanks for asking the quuestion but as I read your question I thought it might be the FL type lenses !?
This F mount lens generation is a killer if it comes to resolution and the better the lens resolution is, the more detail you can recover until - ideally spoken - you hit the wall of sensor resolution.
It may be off-topic related to the thread title but that leads to an interesting question. How much resolution do you really need from your lens to push your camera to the limit ???
Most of the people including Nasim Mansurov @ Photographylife are running their Imatest setups with a D850 which is providing an insane sensor resolution - talking about FX or DX format. But what part of the resolution you can measure with a D850 is actually recoverable by my D750 with its 24 MPixel and thus slightly more than 50% of the D850's sensor resolution, not talking about 21 MPixel of a D5/D6 or even the 16,x MPixel of my D4S ?
@Steve has provided an excellent explanation some time ago for the impact of sensor pixel size to catching motion blur and keeping in mind that if a shivering photographer is not getting sharp images, it is due to motion blur
as well. Against this background it seems logical that the sensor pixel size has major influence on getting sharp images. With all other parameters kept constant you will be able to get sharper images more easy with bigger pixels, but you will not be able to resolve as much detail as with higher resolution sensor, i.e. smaller pixels. At the same time the smaller pixels result in reduced low light tolerance as has been stated above.
Looking at my post above and adding the D850 to the game, its pixel size is 4.35 µm giving a pixel area of roughly 19 µm². This is about 7% moree than the D500 and this fits to the similarity in low light tolerance of both cameras. Of course you can get rid of part of the high ISO noise by downsampling your pictures, but those who use the high res machine for the argument of cropping headroom need to keep in mind that the heavier the crop the more you bring down the effective resolution of you picture and the closer you get to lower res cameras the more obvious their advantage in low light tolerance will be.
And this nothing new and nothing special to the D850 - which I think is a dream macine nevertheless. Years ago my friend (pro photographer) told me baout this just form his experience comparing his D4S as main wildlife and action camera with his D800 and D810 withv the words "the D8x0 is a diva if it comes to avoiding motion blur". Well, if you more than double the sensor resolution and more than half the pixel size you get resolution but you pay for it with low light tolerance and motion blur tolerance. Same story, just one or two camera generations back
.
If I had money like I have grass in the backyard, I probably would have a D6, a D850 and a D500 including all the "FLs" just for having the chance to try everything out
.
But because I am not into architecture or professional landscapes and I usually have to find some things at home to trage in to have the money for another camera or lens, I think I am better of with lower res cameras that are better in low light and have faster frame rate for less money. They deliver stuning results also with lenses that a D850 would push beyond its limits and thus would not be able to fully utilize the camera's capabilities.
Even
@Steve admitted that he is not the most steady person on earth if it comes to handholding, and still sometimes the D850 saves the day by allowing him to crop in cases where he is out there and just doesn't have the right equipment at hand - which I believe only happens very rarely
.
And the other downside with ultra-high res sensors is that they force you in an investment rat race due to the enormous gain in data quantity that is generated and the resources needed to process these pictures in an acceptable way. I checked it for myself and the only bit of my infrastructure that I would not have to extend or even replace when dealing with 45.x MPixel 14bit RAWs would be my monitors. The computer would be far too weak regarding RAM, CPU power as well as local storage and the NAS capacity and performance would need a severe boost as well.
As always the decision is a very individual one and it is good that there are so many different comears out there, so everybody can pick the one that fits her/him best.