Nikon D780 DSLR Camera (Body Only) - Manual (Page 209)

163 Shooting Settings
To focus manually, adjust the lens focus
ring until the image displayed on the
clear matte field in the viewfinder is in
focus.
D AF Lenses
Do not use AF lenses with the lens focus-mode switch set to M and the
camera focus-mode selector set to AF . Failure to observe this precaution
could damage the camera or lens. This does not apply to AF-S and AF-P
lenses, which can be used in
M mode without setting the camera focus-
mode selector to M .