Short for auto exposure lock. It is a button or function that holds the exposure settings the camera has just measured. You point the camera at the area you want to meter, press AEL, and then reframe the picture. The exposure stays the same even though the scene in the viewfinder changes. This is useful when the main subject is not in the center or the scene has strong backlighting. On many cameras the same button can also be set to lock focus (AE-L/AF-L).
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