How to change to brightness of an external display on a macbook pro running OSX 10.6
You cannot do this from System Preferences because the video adapters do not have an interface into the brightness controls of the external monitor.
Sorry, but adjusting manually on the monitor is the only way.
Try the preference pane Shades
There is also Dark Adapted, which will change many more aspects of each screen independently, but it is not longer supported (though it works with lion)
In 2020, there are at least two options:
- Lunar - https://lunar.fyi/
- MonitorControl - https://formulae.brew.sh/cask/monitorcontrol
My setup is a 2013 MacBook Pro with Mojave, connected to two relatively new ViewSonic VA2359 monitors –– one through Thunderbolt, and the other through HDMI.
Lunar successfully changes the brightness of both flawlessly, though the sync option to adjust the externals based on the setting of the built-in display doesn't seem to be reliable. Still, you can adjust both contrast and brightness from a handy menu bar.
I tried MonitorControl, but while it detected both monitors with separate IDs, changing the brightness on either only affected one.