The function (or custom) button: Many DSLRs have a button you can program to perform a single function, such as autofocus lock, depth-of-field preview, ISO, white balance, flash-exposure compensation, drive mode, or dynamic-range control. You program it through the menus, usually the setup folder. What to set it for? Use your camera a lot, and see which control drives you batty from having to rummage through a menu or a jam-packed control panel to get to it. That’s the one to assign to the button. Buttoned down: Several Sony Alpha models, such as the A700, shown here, sport both a function button (which accesses lots of controls) and a custom button (which accesses the one control of your choice).