Nikon D5200: Customize the AE-L/AF-L Button
Set just to the right of the viewfinder, the Nikon D5200's AE-L/AF-L button enables you to lock focus and exposure settings when you shoot in autoexposure and autofocus modes.
Normally, autofocus and autoexposure are locked when you press the button, and they remain locked as long as you keep your finger on the button. But you can change the button’s behavior. To access the available options, open the Custom Setting menu, navigate to the Controls submenu, press OK, and then highlight Assign AE-L/AF-L Button, as shown on the left in this figure. Press OK to display the options shown on the right in the figure.
The options produce these results:
AE/AF Lock: This is the default setting. Focus and exposure remain locked as long as you press the button.
AE Lock Only: Autoexposure is locked as long as you press the button; autofocus isn’t affected. (You can still lock focus by pressing the shutter button halfway.)
AE Lock (Hold): This one locks exposure only with a single press of the button. The exposure lock remains in force until you press the button again or the exposure meters turn off.
AF Lock Only: Focus remains locked as long as you press the button. Exposure isn’t affected.
AF-On: Pressing the button activates the camera’s autofocus mechanism. If you choose this option, you can’t focus by pressing the shutter button halfway.
After highlighting the option you want to use, press OK.
This information with regard to using autofocus and autoexposure assumes that you stick with the default setting. So if you change the button’s function, remember to amend my instructions accordingly.