Quote from iPhone Manual:
"Screen brightness affects battery life. Dim the screen to extend the time before you
need to recharge iPhone, or use Auto-Brightness.
Adjust the screen brightness: Choose Brightness and drag the slider.
Set whether iPhone adjusts screen brightness automatically: Choose Brightness
and turn Auto-Brightness on or off. If Auto-Brightness is on, iPhone adjusts the screen
brightness for current light conditions using the built-in ambient light sensor."