iMacs and MacBooks offer an option to automatically adjust the brightness of the screen based on ambient conditions. This is achieved via a barely visible light sensor near the camera in the top of the screen assembly, and the feature can be activated and deactivated within System Preferences by clicking the Display icon, ensuring the Display tab is selected, and then checking Automatically Adjust Brightness.
Unfortunately, in El Capitan it’s not just the screen brightness (that is the LED backlighting strength) that gets adjusted. Activating this feature will also subtly adjust the gamma of the screen, and this could play havoc if you’re editing photos or videos.
You can test this for yourself in a room with a decent amount of ambient light (that is, a room that’s not dark):
- Open System Preferences, as described above, and ensure that Automatically Adjust Brightness is checked.
- Open a test image in the background so you can watch it for changes.
- Move the brightness slider completely to the right, so the screen is as bright as can be.
- Keep one eye on your test image and uncheck the Automatically Adjust Brightness button. You should see a subtle gamma change happen in the image, particularly in the shadows or darker areas.
Note that you can’t repeatedly test this by simply checking and unchecking the Automatically Adjust Brightness box. To repeat the test, you’ll again need to activate automatic brightness adjustment and decrease and then fully-increase brightness, before deactivating brightness adjustment.
Here’s a mock-up I did to show the subtle difference. Many thanks to Sylvester (a.k.a Captain Sexy Cat) for modelling. The left is WITH automatic brightness adjustment, and the right is WITHOUT. Therefore the image on the right would be the technically true and correct image in this instance:
So, what’s to be done about this aside from simply deactivating automatic screen brightness adjustment – which is actually useful? Wojtek Pietrusiewicz was one of the first to spot the issue and mentions a preferences key he spotted in the Console log: com.apple.AmbientDisplayAgent. I tried the obvious command of defaults write -g AmbientDisplayAgent 0 to attempt to turn off this feature, followed by logging out and back in again, but it didn’t work. However, it’s clear that this can be turned off because the option was there in El Capitan betas – and I’ve a feeling that if enough people discuss it then Apple might return the option in a future update to El Cap.
UPDATE: See comments below where Nicnl details a solution, although this involves a little hardcore system hacking and isn’t something I can recommend.