A8: Af Activation - Nikon D850 Menu Manual

Choose Focus point to enable separate focus-point selection, or
Focus point and AF-area mode to enable separate selection of
both the focus point and the AF-area mode.
Camera rotated Landscape (wide) Camera rotated
90°counter-clockwise orientation 90°clockwise

a8: AF Activation

G button ➜ A Custom Settings menu
Choose whether the shutter-release button can be used to focus
(Shutter/AF-ON) or if focus can only be adjusted using the AF-ON
button or other controls to which AF-ON has been assigned
(AF-ON only). Highlighting AF-ON only and pressing 2 displays
an Out-of-focus release option; select Enable to allow pictures
to be taken without focusing, or Disable to prevent the shutter
being released before the camera has been focused using the
AF-ON control. To take pictures when Disable is selected, use
the AF-ON control to focus and press the shutter-release button
all the way down to release the shutter. Note that Disable only
applies if Focus is selected for Custom Setting a1 (AF-C priority
selection) or a2 (AF-S priority selection) and an option other
than auto-area AF is selected for AF-area mode.
110 Menu Guide