Nikon F 4 Instruction Manual page 42

Autofocus with Optional Autofocus
Speedlights
Although the F4's autofocus works at lighting levels as low as
EV minus 1, performance may vary depending on subject de-
tail and contrast. In dimmest light conditions, use Nikon speed-
lights during AF operation, if it is too dark. The speedlight's AF
Illuminator lights up only in the Single Servo Autofocus mode.
1. Mount Nikon Autofocus Speedlight SB-24, SB-23, SB-22 or
SB-20 onto the F4's accessory shoe.
2. Compose picture through viewfinder and lightly press
shutter release button.
3. Speedlight's AF illuminator lights up and autofocus
operation begins.
•The focal lengths available when using AF Illuminator with AF
Nikkor lenses are 24mm to 105mm.
•For details, see Nikon speedlight instruction manual.