ae loCK

This little button is more useful than you might think. The AE Lock (AE-L) will hold an exposure reading while you recompose your picture. Use the spot metering mode to make a more selective exposure measurement off a tone that you want to set as the mid-tone so the highlights and shadows are reproduced as desired. The AE Lock means you can make this measurement from any desired point on the subject and then recompose. As shown here (on the Nikon D610) some cameras allow for this button to be configured at either the AE Lock or the Autofocusing Lock (AF-L) or to perform both functions simultaneously.