The Shifting Illumination
Modern MacBooks and iMacs are equipped with ambient light sensors and a feature Apple calls “True Tone.” True Tone automatically adjusts the color temperature of your display to match the lighting in your room, while the auto-brightness feature constantly shifts the backlight intensity based on shadows or sunlight hitting the sensor. While these features make reading text more comfortable in variable lighting, they are an absolute nightmare for creative professionals. If you are grading video footage, editing photography, or designing a website, having the screen constantly change its brightness and color balance completely destroys your ability to accurately judge colors and contrast.
Locking the Screen Calibration
To establish a consistent, reliable visual baseline and stop macOS from automatically changing the display brightness and color temperature, you must disable both the ambient sensor response and the True Tone algorithm.
Click the Apple logo in the extreme top-left corner of your screen and select System Settings. In the left-hand navigation sidebar, scroll down and click on Displays. On the right-hand side, look for the “Brightness” slider. Immediately below that slider, you will see a toggle switch labelled Automatically adjust brightness. Click this toggle so it turns grey (Off). Next, look slightly further down for the toggle switch explicitly labelled True Tone. Click this toggle to turn it Off as well. Your Mac display is now completely manual. The brightness will remain locked at whatever level you set the slider, and the color temperature will remain perfectly static, regardless of the lighting in your environment.