How to control brightness on external late 2009 iMac monitor when connected to new 2012 iMac primary monitor? I have a late 2012 27" iMac as my primary monitor also connected to a late 2009 27" iMac as a secondary monitor via a mini-display port to mini-display port cable. I also have a Dell 24" monitor connected to the late 2012 27" iMac as a third monitor.

I can change the brightness on the Dell monitor manually with the Dell hard buttons but I cannot change the brightness on the late 2009 iMac using the keyboard attached to the late 2012 iMac

Is there anyway to change the brightness on the iMac which is serving as a secondary display?

Other details:


I don't want to use the software Shades as it’s a memory hog and leaks memory
I'm using a wired Mac keyboard connected via usb to the late 2012 iMac
control + F1 or control + F2 does not work.
I have the same situation! Only thing that works so far is a second keyboard connected to the late 2009 iMac, but that's quite annoying according to my available workspace on my desk.