I am trying to use my 2009 iMac 27" as a second display for my 2013 MBPr 15" (both running El Capitan). I have connected the two with a thunderbolt cable and everything is working fine except one thing – once the iMac starts acting as a display for the MacBook, it switches to full brightness. When I open up display preferences, the brightness slider that is usually there is gone.
I have seen other people online who had this problem years ago, but was unable to find a solution. Is there any way to adjust the brightness on the iMac when using it as a second display?