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: Go to Settings > Brightness and drag the slider.
Set whether iPhone adjusts screen
Go to Settings > 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.