Turning off ambient light control?

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?

System Preferences → Displays → Automatically adjust brightness

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.