Auto-brightness is a feature designed to save battery life and make your iPhone’s screen easier to read. When you’re somewhere dark, like your bedroom at night, the screen will automatically dim. When you’re somewhere bright, like outside on a sunny day, it’ll crank itself up to maximum brightness.
Auto-brightness is generally pretty good at adapting to changing circumstances, and since the screen is one of the biggest battery drainers on your phone, we’d generally recommend leaving it on. However, if you’re someone who prefers to control it manually, or finds it consistently misses the mark when it comes to the brightness you’re looking for, here’s how to turn it off.
Head to Settings > General > Accessibility > Display Accommodations.
Toggle the Auto-Brightness switch to off.
Now the screen brightness will remain constant no matter the light level of your location. If you want to change the brightness, swipe up from the bottom of your screen to access the Control Center (which just got a big update in iOS 11) and adjust the brightness slider. Just be careful not to blind yourself with a really bright screen at night!