Brightness

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.