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I have a mid-2011 27" iMac. I have realized that the screen brightness adjusts depending on the ambient light. When the sun hides behind the clouds, and the room gets darker, the brightness decreases. I have located the sensor to be the camera. When putting a finger over the camera on top of the screen I can reproduce the sun hiding.

How do I turn this magic feature off?

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3 Answers 3

up vote 6 down vote accepted

System Preferences → Displays → Automatically adjust brightness

enter image description here

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Thanks! I really tried to google for this answer but didn't find anything relevant. StackExchange is the greatest. – matpe Mar 10 '12 at 14:51

In System Preferences, go to the Displays section, under Hardware. There is a slider to control brightness, and right under it (or next to it) there is a checkbox to control automatic brightness adjustment.

enter image description here

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I have a mid-2010 27" iMac, using OS Lion. The "Automatically adjust brightness" checkbox does not appear in Displays. – user87928 Aug 16 '14 at 20:28
@MikeSchneider Before 2011, iMacs were not equipped with an ambient light sensor, so they can't measure brightness levels. Sorry! – timothymh Aug 28 '14 at 21:47

Rubbish, my iMac early 2006 has a light sensor and it can be disbled in prefernces...

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