Film Speed Dial; Controls In Detail - Nikon F 4 Instruction Manual

CONTROLS IN DETAIL————————————————————————————

FILM SPEED DIAL

The F4 offers two ways to set film speed, depending on film in use.
DX-Coded Film
While pressing the lock release, set the film speed dial to DX.
Camera automatically senses film speed (ISO 25 to 5000) of
installed DX-coded film.
With dial set to DX, use only DX-coded film. If non-DX-coded
film or film with a defective DX code is loaded, or no film is
loaded, lightly pressing the shutter release button causes the
red alert LED to blink, and the shutter locks.