QuickShade

Available from the Mac Apple Store

When you connect an Apple display up to a Mac or MacBook the brightness controls built in to the functions keys, dims both the MacBook screen and the attached Apple screen. The same keys on an Mac / MacBook will dim the internal display but a second none Apple display connected will stay at its native brightness.

What quickshade does (by fooling the display out signal) is reduce the external display brightness to match the built in displays. very clever and super simple.

The brightness keys will work for the built in display, allowing you to tweak the external displays brightness using QuickShade in the located in the menu bar, within the apps preferences allows you to set shortcut keys to adjust the brightness, and the standard option to open at login.