Nikon D300 User Manual (en)

f6: Assign AE-L/AF-L Button
Choose the role played by the AE-L/AF-L 
button, either by itself ([AE-L/AF-L button 
press]) or when used in combination with the 
command dials ([AE-L/AF-L+command 
dials]). The options available for [AE-L/AF-L 
button press] are the same as for [FUNC. 
button press] (pg. 302), except that [AE-L/AF-L button press] 
defaults to [AE/AF lock] and has an additional 
B option (if this 
option is selected, pressing the AE-L/AF-L button has the same effect 
as pressing the 
B button to initiate autofocus). The options 
available for [AE-L/AF-L+command dials] are the same as for 
[FUNC. button+dials] (pg. 304), except that [AE-L/AF-L +command 
dials] defaults to [None] and lacks [1 step spd/aperture] option.