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I have a Windows7 64-bit machine attached to a 27" iMac as an external displayport monitor It's working well, except I cannot adjust the brightness.

I have attached a Mac keyboard both on the Mac & Windows side to gain access to the brightness controls, with no luck. As soon as I switch to display target mode, the screen jumps to full brightness, and cannot be controlled from the keyboard on either platform.

I have searched the web but have not found the answer:

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possible duplicate of Can I change the brightness on a secondary display? – daviesgeek Mar 15 '12 at 16:41
Using an AppleKeyboard won't help since you don't have the drivers for it's function keys. Maybe installing bootcamp drivers on your machine will help. – Kevin Grabher Apr 11 '15 at 9:42

One poster in this Apple discussion forum had a suggestion:

I've futzed with this for a while using a MBP and an iMac, both running 10.6.8. The problem is inconsistent for me, but seems to be linked to how long I've had the iMac on. If on chronically (as I usually leave it), the iMac tends not to respond to F1/F2 brightness keystrokes when I connect my MBP and use the iMac in target display mode. But if I reboot the iMac and then connect from my already running MBP, this restores brightness keystroke response. That is:

  1. With MBP and iMac running, connect mini DP to start target display mode on iMac
  2. If brightness keystrokes don't respond, disconnect mini DP and reboot iMac, leave MBP running
  3. After booted to desktop on the iMac, reconnect mini DP - brightness keys should respond

I don't have your setup, so I can't verify that this solution works. However, it's worth a shot.

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I did everything you said replacing my Windows machine for the MBP in your setup. No dice :( – zen Mar 16 '12 at 14:29

It is probably easier to adjust the brightness in windows from the display drivers settings. usually you can right click empty desktop and the display drivers UI is the first entry in context menu (like "AMD Vision Control Center"). All three major video card vendors can adjust brightness on the drivers side.

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It is confirmed that should be able to control that with iMac's keyboard. I've tried but it doesn't work in your setup.

Also, the brightness doesn't show up under OS X's System Preferences, and I have no chance to test it under Windows since my Thunderbolt Macs will only connect under OS X, not under Windows 8 Boot Camp.

I will try DDC/CI software though it have a high chance of not working.

Your last option, though bad, is to adjust it with software like how you adjust it in game.

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