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Find the setting for “Auto-Brightness” and toggle OFF or ON as needed.
Open the “Settings” app and go to “General” and then to “Accessibility”. The auto-brightness setting has a new home in iOS 11 & iOS 12 onward, now contained within the Accessibility section of the Settings app, here is where it can be found: How to Turn OFF or ON Auto-Brightness in iOS 12 on iPhone and iPad This has led some users to think that auto-brightness was removed in iOS 11 and iOS 12, but in fact the setting was merely relocated. However, as of iOS 11 and iOS 12, the auto-brightness setting has been relocated from the standard Display settings area to deeper within the settings of iOS.
Some users like to turn off auto-brightness, or to make sure that auto-brightness is enabled on their iPad or iPhone.