Automatic exposure lock. This is a push-button control that enables you to select the part of the scene from which the camera takes its meter reading, and then lock this setting while the image is re-framed for better composition. The auto-exposure lock (AE-L) function on a DSLR 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. Just about all DSLR cameras have an auto exposure lock button.