Unable to control brightness in target display mode

Kelvin -- Same here, connecting a 2011 MacBook Pro to a 2009 iMac 27" and using Target Display Mode. The volume adjustment keys work, but not the display brightness. This used to work prior to 10.6.8...

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>

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.

I had the same problem on my 27" iMac (late 2010) under OS X 10.6.8.

Sadly, upgrading to Lion still hasn't fixed the bug.

This bug is still apparent the latest developer builds of Lion. I wonder if this means a fix won't appear in 10.7.1.

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.

I've the same issue with 10.7.0 on iMac and MacBook Pro.

Exactly same problem is here. I have upgraded to Lion OS. Apple, please fix it on priority basis.

Updated my iMac to 10.7.1 and the bug is still there. However, a sign of hope: 10.7.2 may fix this annoying issue. See post #11 here: http://forums.macrumors.com/showthread.php?t=1177587

For others wanting a fast reply, the thread above indicates this is fixed in OSX Lion 10.7.2. This update has the iCloud bits in it so it will be out within a week or two at the most.

I've installed 10.7.2 on my iMac. It has fixed the problem : I can now change the brightness using the iMac keyboard when I use it as target display for my MacBook Pro.

Yeaah, finally !

I installed 10.7.2 on both iMac and MacBook Pro. The problem persists for me.

It started with 10.6.8 as stated above, full brightness no controls :-(

I am stuck having to keep 10.6.8 because I have an irreplaceable peripheral that will never go beyond Snow Leopard. This issue of Imacs in target mode has gone on for a long time. Apple has fixed it in Lion but this isuue has existed in SL since 10.6.7. PLEASE APPLE!! Fix this.

As mentioned, it's not fixed in Lion for me (even though some people reported so). Anybody else having the problem persist in Lion?