Nikon D3000 Reference Manual page 153

AE-L/AF-L
Choose the role played by the AE-L/AF-L button.
.
Option
B AE/AF lock
C AE lock only
F AF lock only
E AE lock (hold)
A AF-ON
AE Lock
At the default setting of Off, exposure only locks when the AE-L/
AF-L button is pressed. If On is selected, exposure will also lock
when the shutter-release button is pressed halfway.
Focus and exposure lock while the AE-L/AF-L button is pressed.
Exposure locks while the AE-L/AF-L button is pressed.
Focus locks while the AE-L/AF-L button is pressed.
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.
The AE-L/AF-L button initiates autofocus. The shutter-release button can
not be used to focus.
Description
AE-L/AF-L button
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