The auto exposure lock (AE-L) function on a D-SLR camera lets you physically lock the exposure reading from anywhere in the scene. You can use it on its own or at the point where you focus the image.
All digital SLR cameras have an auto exposure lock button. When you press the AE-L button, the current exposure settings are fixed (locked) so that they can’t change as you recompose the shot – even if the level of light alters as you aim the camera elsewhere.