X setting – Nikon F4 User Manual
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“X” setting provides a top flash sync speed of 1/250 sec. with
Nikon speedlights. For assurance during flash shooting, set the
shutter speed dial to "X”. To unlock, while pressing the lock
release button, shift the shutter speed dial to another setting.
Speedlight synchronization is also possible for dial-selected
speeds from 1/250 sec. and slower.