This button AEL or * appears on the majority of DSL cameras but 90% of people never use it and dont even know what its for. This will explain what it does and how to use it and knowing this will be useful, because you will need this function at some point.
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. This is where the auto exposure lock function comes in handy. It lets you take an exposure reading independently of where you want to focus. The AE-L button can be used if the subject or subjects are positioned off-center in the scene. It is also useful when you’re shooting a sequence of images that you want to be stitched together in order to produce a panoramic photograph. The AE-L function makes sure that every shot taken in the sequence has exactly the same exposure settings.