Nikon 5000 - Manual (Page 164)

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Camera Setup
Options for the AE-L/AF-L Button: AE-L, AF-L
By default, both focus and exposure are locked
when the AE-L/AF-L button is pressed. The op-
tions in the AE-L, AF-L menu can be used to set
the button to lock only one of focus or exposure.
Option Description
AE-L&AF-L
Pressing the AE-L/AF-L button locks both focus and ex-
posure.
AE-L
Pressing the AE-L/AF-L button locks exposure only. Focus
is locked when shutter-release button is pressed halfway.
AF-L
Pressing the AE-L/AF-L button locks focus only. Exposure
is locked when shutter-release button is pressed halfway.