Nikon D300 Complete Manual page 581

Button Press only:
Preview
FV lock
AE/AF lock
AE lock only
AE lock (Reset)
AE lock (Hold)
Thom Hogan's Complete Guide to the Nikon D300
The DOF Preview button acts as a
DOF Preview button [default]
The flash is preflashed once and the
value for flash locked when the DOF
Preview button is pressed; a second
press cancels FV lock [default]
Holding the DOF Preview button
locks both exposure and focus
Holding the DOF Preview button
locks exposure
Holding the DOF Preview button
locks exposure, but releases the lock if
you press the button again, the meter
times out, or you press the shutter
release
Pressing the DOF Preview button
locks exposure until the meter times
out or you press the button again
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