Thanks to light sensors, your iPhone can automatically adjust the display brightness to match the ambient lighting. If you prefer manual controls, it’s possible to turn off Auto-Brightness, but Apple put the option in an unusual place.
Strangely, the iPhone and iPad’s automatic brightness option is not in the “Display & Brightness” settings like you’d expect. You’ll find the “True Tone” toggle, but nothing for Auto-Brightness. The good news is it’s not hard to find, you just need to look somewhere else.
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Here’s where Apple throws the curveball. You actually want to go to “Accessibility,” not the Display settings.
Scroll down to the bottom and toggle off the switch for “Auto-Brightness.”
Now when you adjust the brightness it will stay at the level you set until you change it again. This can be a good trick to save battery life—if you keep the brightness low—or it can drain the battery quickly if you leave it on high brightness a lot.