The Ambient Light Sensor
MacBooks and Apple displays are equipped with an ambient light sensor built into the bezel near the camera. By default, macOS uses this sensor to automatically adjust your screen’s brightness to match the lighting conditions of your environment. If you move from a dark room to a bright window, the screen gets brighter. If a cloud passes over the sun, the screen dims. While this saves battery life and reduces eye strain, it can be incredibly frustrating for photo editors, graphic designers, or anyone watching a movie who requires a consistent, unchanging level of screen brightness.
How to Disable Automatic Brightness Adjustment
You can turn off the ambient light sensor entirely, forcing macOS to respect the exact brightness level you set using your keyboard or control centre.
1. Click the Apple logo () in the top-left corner of the menu bar.
2. Select System Settings (or System Preferences on older versions).
3. Scroll down the left-hand sidebar and click on Displays.
4. Look for the brightness slider at the top of the right-hand pane.
5. Directly beneath the slider, you will see a toggle switch labelled Automatically adjust brightness.
6. Click this toggle switch to turn it Off.
Managing Battery-Saving Dimming
There is a secondary setting that causes your screen to dim independently of the ambient light sensor: the battery saver. If your MacBook is running on battery power, macOS may slightly dim the screen just before the display turns off to save energy. To disable this behaviour as well, go to System Settings > Displays > Advanced… and untick the option for Slightly dim the display while on battery power. Your screen will now remain at maximum brightness until the moment it goes to sleep.