Mac-Monitor dimming
- Press the keys "F2" or "F3" to decrease the brightness or to increase it. You want to change the brightness in smaller steps, hold the keys "alt" and "SHIFT" pressed.
- Alternatively, you can adjust the brightness in the system settings. To do this, click under "Hardware" to "monitors" and move the Slider in the desired direction.

Brightness in the settings
Monitor on a Mac's auto dim
Case, you do not want to adjust the brightness manually, just enable auto-brightness:
- Start the "system settings".
- Here, you select the menu item "monitors".
- Now a box under the Slider for the brightness. Here you can set a hook, in order to have the brightness automatically in the future regulate.

Automatic Brightness
With the Dimmer to alter the brightness
"Dimmer" is a special program with which you can regulate the Monitor brightness and the colors of the screen can change:
- Download the program here download.
- You start it then. In the upper task bar is a light bulb icon appears.
- By clicking on the sign set the brightness and the colors of your monitor. In the color settings, you should be careful, though, because it can cause strong and unwanted color changes.

"Dimmer" for MacOS
How to use the volume on Mountain Lion fine-tune, you can learn here.