The Shifting Screen
Modern MacBooks feature a highly advanced ambient light sensor located near the webcam. macOS utilizes this sensor to constantly monitor the lighting conditions in your room. If a cloud passes over the sun, or you turn off a desk lamp, the operating system will automatically and smoothly dim the display brightness to conserve battery and reduce eye strain. While usually helpful, this constant shifting of brightness is unacceptable if you are performing color-critical work like photo editing or video grading, where the perceived contrast of an image changes based on the screen’s backlight intensity.
Locking the Backlight
To ensure consistent visual accuracy and stop macOS from automatically dimming the display based on ambient lighting, you must disable the auto-brightness feature in System Settings.
Click the Apple logo in the top-left corner of the screen and select System Settings (or System Preferences on older macOS versions). Scroll down the left-hand navigation sidebar and click on Displays. In the main right-hand panel, you will see a slider controlling your current brightness. Look immediately below this slider for a toggle switch labelled Automatically adjust brightness. Click this toggle to turn it Off (the switch will turn grey). Furthermore, if your Mac supports it, you should also disable True Tone (which automatically shifts the screen’s color temperature based on ambient light) to ensure your screen remains perfectly static and color-accurate regardless of your environment.