Change brightness on iMac in target display mode

Recent iMacs support a "target display" mode, where you can connect another Mac using a DisplayPort cable, and the iMac will function as an external monitor.

This used to work fine, however now if the iMac is running Mac OS 10.6.7 or later (including Lion), the iMac screen will switch to 100% brightness when put into this mode, and the brightness keys on the iMac keyboard can not be used to adjust the brightness. Has anyone found a solution to this problem?

Solution 1:

yeah, toggling the function key setting worked for me. I'm on 17" 2011 mbp and the 27 inch cinema display with Lion.

Solution 2:

Ctrl-F1 (which has a "dimming" circle with short marks around it) or Ctrl-F2 (which has a "bright" circle with longer marks around it)work on the external display. Dim and Bright, alone, work on the native display.

Solution 3:

The brightness keys would sometimes work and sometimes not on the target iMac. So I solved this with a System Preference, Shades for Mac. It puts a set of sunglasses in your toolbar and you can easily adjust the brightness.