Nikon F4 - Manual (Page 121)

VIEWFINDER ILLUMINATOR SWITCH
When it’s dark, use the viewfinder illuminator to light up all
viewfinder information. Turn the switch on, and lightly press the
shutter release button to illuminate the display. The illuminator
automatically switches off as the viewfinder display disappears;
it also momentarily switches off during exposure. Although the
illuminator is automatically switched off when the camera is set
to the L (lock) position, switch it off, when no longer needed, to
conserve power.
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