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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?

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Sadly this problem still exists. My work around is as follows

1) Apple F2 to share displays. The screen is usually quite dark.
2) Apple F2 to switch back to iMAC display.
3) I can now adjust the brightness on the iMac, I set it to my desired level.
4) I Apple F2 back to the shared display I initially wanted.

It is indeed quite annoying and generally a bad experience.

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I changed the the thunderbolt port on MBP(2015) (same port on iMac(2014) and same cable) and it was solved.

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