The iPhone automatically adjusts screen brightness based on ambient lighting, but it doesn't work as well as some would like. Luckily the ambient light sensors can be manually adjusted so that they behave correctly. By following a few easy steps, Auto-Brightness can be reconfigured to correctly respond to changing light conditions.
Users can switch Auto-Brightness off completely if they wish, but we recommend spending a couple of minutes resetting it and leaving it on for the best experience and to prevent eye fatigue.
How to Fix Auto Brightness
To properly configure the Auto-Brightness of your iPhone you will need both a bright flashlight and a dark room. These two things help to configure the light sensor so that the iPhone will adjust correctly