The Buttons option on the Setup menu offers a third button tweak called Shutter-release Button AE-L, as shown in the following figure. This option determines whether pressing the shutter button halfway locks focus only or locks both focus and exposure. (AE-L stands for autoexposure lock.)
If you turn this option on, pressing the shutter button halfway locks exposure and focus.
At the default setting, Off, you lock focus only when you press the shutter button halfway. Exposure is adjusted continually up to the time you take the shot. If you change the Shutter-release Button AE-L setting to On, your half-press of the shutter button locks both focus and exposure.
