The Monitor Mode Button And The Eye Sensor - Nikon Z 8 Reference Manual

Tip: Using the Viewfinder with the Camera in "Tall" Orientation
In video mode, the playback and playback i menu displays rotate
to match camera orientation.

The Monitor Mode Button and the Eye Sensor

Placing your eye to the viewfinder activates the eye sensor,
switching the display from the monitor to the viewfinder. Note that
the eye sensor will also respond to other objects, such as your
fingers.
The viewfinder can be used for menus and playback if desired.
Press the M (monitor mode) button to cycle between viewfinder
and monitor displays.
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The Viewfinder