Nikon D850 User Manual page 126

AF-area mode can be selected by pressing the AF-mode button
and rotating the sub-command dial until the desired setting is
displayed in the viewfinder and control panel.
AF-mode button
A
3D-tracking
When the shutter-release button is pressed halfway, the colors in the
area surrounding the focus point are stored in the camera.
Consequently 3D-tracking may not produce the desired results with
subjects that are similar in color to the background or that occupy a
very small area of the frame.
102
Sub-command
dial
Control panel
Viewfinder