All you have to do to turn off auto-brightness is go to Settings > Display and turn off the adaptive brightness feature (via Google 9 to 5). So why do you need to turn it off, anyway? It is recommended that you change your phone from its default brightness setting so that you can control how bright you want your phone to be. This is mainly for two reasons. The first is battery life. Bright light helps you see your screen better, especially if you’re outdoors, but it will quickly kill your battery. If you want to reap the benefits of long battery life, it’s probably best to turn the screen brightness down and only turn it up for those times when you need more light. A good rule of thumb is to manually adjust your screen brightness to the brightness level of the room you’re in.
The second reason has to do with protecting your eyesight. Like all smartphones and other electronic devices, your Android phone emits blue light to help you see the display screen. But this light can damage the retina if you spend long periods staring at your phone. That’s why it’s a good idea to turn off auto-brightness to maintain healthy eyesight and reduce eye strain.
