First secret phrase:
ISO sensitivity settings. This is found in the shooting menu. See pages 252 and 119 in the D850 manual for different settings and ways to set ISO. Also as noted pressing the ISO button and rolling the main command wheel changes the ISO. Rolling the sub-command wheel changes between manual and auto ISO. There are more options in the Shooting menu> ISO sensitivity settings> dialogs.
Next:
Image area is found in the shooting menu and on page 251 in the D850 manual. If you search the PDF manual there are many hits for Image area and there are hits for
still and movie photography so you need to read the correct page for the mode you are using. There are a lot of hits to wade through. If the
Shooting menu> Choose image area> is set to DX or something other than FX you'll get what's set regardless of what lens is on the camera.
The manual can be quick and easy or if the manual is in a bad mood it can be slow and tedious. I remember trying to find "Slave" in the SB-800 manual and got zero hits. I may have tried "Master" and found on the same page found the newer terms which are "Commander" and "Remote". The index is not redundant so "slave" gets zero hits and that's that.
I'm not sure anyone loves the User's Manual. For me it's a necessary evil. I probably use trial and error first.
Hope I covered this, I hope this is helpful.
Dave Hartman
I need to mention that typos don't count when using Adobe Reader, Advanced Search.