Quick question last weekend i borrowed a D850 to use alongside my D4 anyway the owner of the D850 was unaware as i was that everytime you change a lense the camera reverts back to 37.1 mp with a 1.2 crop. Annoyingly thou it seems the view finder shows the whole pic so we ended up with a few footless people. I did some digging and it seems you can disable it doing it. Anyone know how? does seem abit of a weird thing for them to make it do thou. I went throu the menus on the camera and i couldnt see any option to disable it or come to that revert it back to its 45 mp.
Try page 84 of the manual. I have never owned a Nikon but Martin or Des should know. It seems that you can set it so if you connect a cropped lens, it adjusts automatically which is probably right as long as it reverts automatically as well.
The thing is i was using all prime full frame lenses my D4 will automatically crop it if i put a DX lense on it which you can in the menu turn the option on or off. On the D850 it had a 35 1.4 prime and a 85 1.4 prime and it stayed at 1.2 crop factor even thou it was set to the largest Raw file size which was 37.1 mp when the sensor is 45 mp.