Nikon D90 - Manual (Page 220)

200
L
f4: Assign AE-L/AF-L Button
Choose the role played by the AE-L/AF-L button. This option is
available in all shooting modes.
.
Option Description
B
AE/AF lock
(default)
Focus and exposure lock while the AE-L/AF-L button is pressed.
C AE lock only Exposure locks while the AE-L/AF-L button is pressed.
F AF lock only Focus locks while the AE-L/AF-L button is pressed.
E AE lock (hold)
Exposure locks when the AE-L/AF-L button is pressed, and remains locked
until the button is pressed a second time or the exposure meters turn
off.
A AF-ON
The AE-L/AF-L button initiates autofocus. The shutter-release button can
not be used to focus.
r FV lock
Press the AE-L/AF-L button to lock flash value (built-in flash and SB-900,
SB-800, SB-600, SB-400, and SB-R200 flash units only, pg. 198). Press
again to cancel FV lock.