Nikon F4 Instruction Manual page 43

Freeze Focus
The optional Nikon Multi-Control Back MF-23 or 250-Exp.
Multi-Control Back MF-24, when attached to the F4, offers a
Freeze Focus function which automatically fires the shutter
when the subject enters a preset focus position. This feature is
especially convenient for manual focus with the F4's Electronic
Rangefinder. For details, see the MF-23 or MF-24's instruction
manual.
43