Your iPhone is equipped with a highly sensitive ambient light sensor hidden inside the top bezel (or the Dynamic Island). This sensor constantly monitors the light in the room around you. If you step outside into bright sunlight, the screen automatically cranks to maximum brightness so you can read it. If you step into a dark movie theater, the screen instantly dims so you do not blind everyone around you.
While this is generally a fantastic battery-saving feature, it can be infuriating if you are trying to edit a photograph or watch a dark movie, and the screen keeps randomly dimming because a shadow passed over the sensor. Apple intentionally hides the switch to turn this feature off, assuming most users will ruin their battery life if they touch it.
The Hidden Switch is Not in Display Settings
The most confusing aspect of this process is that the master switch is not located in the obvious “Display & Brightness” menu. If you look there, you will only find the manual slider and the True Tone toggle. Apple moved the master switch deep into the Accessibility menu years ago.
- Open the Settings app on your iPhone.
- Scroll down to the third main block of settings and tap on Accessibility (the blue icon with a stick figure inside a circle).
- In the “Vision” section at the top, tap on Display & Text Size.
- Scroll all the way down to the absolute bottom of this incredibly long menu.
- You will see a master toggle labeled Auto-Brightness.
- Tap the green switch to turn it Grey (Off).
How to Manually Control Your Screen
Now that the automated sensor is deactivated, the iPhone will absolutely never change the brightness of the screen on its own. If you set the screen to maximum brightness, it will stay at maximum brightness until the battery dies.
You must now control the brightness entirely manually using the Control Center.
- Swipe your finger diagonally down from the top right corner of the screen (where the battery icon is).
- Look for the large vertical slider icon with a Sun symbol inside it.
- Drag your finger up the slider to make the screen brighter, or drag it down to make it dimmer.
Why Does My Screen Still Dim When It Gets Hot?
Even if you successfully turn off Auto-Brightness in the Accessibility menu, you may still experience a phenomenon where the screen suddenly drops to 50% brightness and refuses to go any higher, even if you push the manual slider all the way up.
This is not a software glitch. This is Thermal Throttling. If you are playing a heavy 3D video game outside in the summer sun, the iPhone’s processor generates massive amounts of heat. To prevent the physical internal circuitry from melting or the battery from catching fire, the iPhone forcibly cuts power to the screen.
You cannot turn this safety feature off. The only way to restore maximum brightness is to stop using the phone, move it out of the direct sunlight, and wait 15 minutes for the glass to physically cool down.