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Unable to control brightness in target display mode

I use my 27" mid2010 iMac as a display for my MacPro. However after the Mac OS X recent update to 10.6.8, I noticed I am unable to control the screen brightness. Usually, I was able to do so using the iMac keyboard to adjust brightness etc. but now they do not respond in Target Display Mode. The brightness adjustment do respond if I am using the iMac but not anymore in Target Display Mode. This is a little frustrating as the screen is really bright and is hurting my eyes.

Posted on Jul 1, 2011 8:37 AM

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25 replies

Jul 4, 2011 7:53 AM in response to KelvinLeFabuleux

Same here. Late 2009 iMac 27". This must be a bug. (Apple: please fix this, thanks!)

As a temporary solution I installed one of the brightness control apps you can find in the App store.

I used Midnight from Ollix, searching for 'brightness' in the app store reveals some more.


<note: controlling brightness this way affects colors and is no REAL fix, just makes your eyes hurt less until the bug is fixed>

Jul 7, 2011 1:01 AM in response to KelvinLeFabuleux

Same issue. 27" mid 2010 iMac and 2011 MBP. Full brightness but it started first with target display getting lost : iMac display was not recognized on the MBP. CMD-F2 was just flashing the screen black and nothing. Had to reboot iMac to enjoy the 27" full brightness.

Same issue using Mac Pro with HD5870 as a source.

Audio still working as expected, through Display Port, and through Line In for the Mac Pro.

Jul 28, 2011 9:11 PM in response to KelvinLeFabuleux

This bug was introduced in 10.6.7 with the addition of Thunderbolt. Until Apple bother to fix their mess, the only solution I have found is to restart the iMac repeatedly until you are able to control the brightness in target display mode. FYI, I use target display mode with an AMD Radeon 6990 connected to my iMac, your result may vary with a Mac to Mac.

Unable to control brightness in target display mode

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