The Shifting Backlight
Modern MacBook displays are equipped with a built-in ambient light sensor located near the webcam. By default, macOS uses this sensor to actively monitor the lighting conditions in your room. If a cloud passes in front of the sun, or if you turn off an overhead light, macOS will automatically detect the change and aggressively dim or brighten your screen to compensate. For creative professionals doing colour-critical work like photo editing or video grading, this automated adjustment is a nightmare. As the screen brightness fluctuates throughout the day, your perception of contrast and saturation is constantly skewed, making accurate editing impossible.
Locking the Display Brightness
To ensure your monitor output remains absolutely consistent and stop macOS from secretly altering your display settings in the background, you must disable the automatic brightness adjustment.
Click the Apple logo in the top-left corner of your screen and select System Settings. Scroll down the left-hand navigation pane and click on Displays. In the main right-hand panel, look just below the brightness slider. You will see a toggle switch labelled Automatically adjust brightness. Click this toggle to turn it completely Off. Additionally, if your Mac supports True Tone (which automatically alters the colour temperature based on ambient lighting), you should also turn the True Tone toggle Off to ensure maximum colour accuracy. From now on, your screen brightness will remain exactly where you manually set it using the keyboard shortcut keys, regardless of how the lighting in your environment changes.