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I have periodically noticed that my display has been a bit dim, and have brought it up to maximum brightness... only later to find that my display was about halfway on brightness. I can't rule out a stray keystroke, but it seems to be several notches down a lot more often than I would accidentally hit F1. As far as I can tell this happens when the computer remains on and I remain logged on.

Especially when operating on battery power, does Mountain Lion dim the display automatically? Is there a way to turn off or circumvent this?

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I actually find the opposite frustrating: I typically use my computers at near-minimum brightness and it's very annoying when I reconnect to power or go into a bright room and the display is suddenly blaring with light. – CajunLuke Sep 19 '12 at 21:14

When you go from mains power to battery, OS X decreases display brightness. (It should go back to normal brightness when you reconnect to mains power.)

To turn this off, go to System Preferences > Energy Saver, choose the Battery tab, and un-check "Slightly dim the display when using this power source".

The display can also change brightness because it senses a change in ambient brightness. To turn this off, go to System Preferences > Displays, find the window on the display you want to change this setting on, go to the Displays tab, and un-check "Automatically adjust brightness".

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