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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:
http://support.apple.com/kb/HT3924
http://macs.about.com/od/usingyourmac/qt/using-target-display-mode.htm
https://superuser.com/questions/60145/how-do-i-use-a-new-27-imac-as-an-external-pc-monitor-with-full-2560x1440-resolu

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11 Answers 11

Reset to default
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I just had an issue with not being able to change the brightness on my 27inch iMac in Target Display Mode. It was at full brightness. And the brightness keys on the keyboard hooked up to the TDM Mac would not adjust it. But I found if I pressed and held Commond-F1 on that keyboard it slowly lowered the brightness. This probably won't help much if you have an issue with it defaulting to a low brightness though.

iMac 2009 High Sierra and iMac Pro Mojave.

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I had the problem of brightness stuck on maximum when using a 27" late 2009 iMac (running High Sierra macOS 10.13.6) as an extra screen (in target display mode) using a MacBook Pro 15in (2018) running macOS Catalina 10.15.3. The iMac had an Apple wireless keyboard connected, and the macBook Pro was connected to the iMac via a USB-C to mini displayport cable (i.e. in target display mode). The display on the iMac toggled from the iMac in the normal mode to being an extra display for the macBook using command-F2 on the wireless keyboard. I found that the brightness control keys on the wireless keyboard (F1 and F2) for the iMac in target display mode only worked when I was logged into an account on the iMac. If the iMac was restarted, and nobody was logged into it, the brightness keys on the wireless keyboard did nothing. So when you start the iMac you need to log in to it before putting it into target display mode.

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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
    Commented Mar 16, 2012 at 14:29
  • Yes, that works when I connect my iMac in TDM with my MacBook Air. However, I do not need to reboot the iMac. Simply reconnecting the cable works well for me. Another trick that works "many times" is to press Command+F1 (while the lid of the MacBook is open) and enable extended desktop mode. Once in that mode, the iMac starts responding to the brightness keys after certain attempts, and continues to do so after coming out of extended desktop mode (by pressing Command+F1 again).
    – AmanNoug
    Commented Jul 11, 2017 at 19:56
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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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I'm running a late 2009 i7 27" iMac in Target Display mode connected to a 2018 MBP. I could not control the brightness on the iMac display using the keyboard running to the MacBook Pro, BUT, by connecting a second keyboard directly to the iMac, I found that the brightness controls on that second keyboard DID allow me to adjust the brightness. Hope that helps....

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There's lots of good software programs listed under this question: http://apple.stackexchange.com/questions/20237/can-i-change-the-brightness-on-a-secondary-display

But for a no-install solution, I've used the new "night shift" feature to dim things. Works if you just need to take the edge off on the brightness. In System Preferences > Displays > Night Shift you can adjust the Color Temperature slider to change how much effect it has.

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I was having this problem, too. What I found is that when mirroring the displays the external keyboard adjusts brightness on both computers. When NOT mirroring the displays the keyboard on the primary Mac (in my case a Macbook Pro) adjusts the it's own display and the external keyboard connected to the MacBook Pro adjusts the iMac display.

I am running Sierra (10.12.6) on a 2012 27" iMac and El Capitan (10.11.6) on a 2012 15" MacBook Pro.

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I wound up using: Brightness Slider, free in the Mac App store.

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Hoping this will help someone..

I was having the same issue. Brightness control keys on the keyboard connected to the iMac would sometimes work and sometimes would not. I was thinking this is a bug and the whole Target Display Mode is just glitchy, but I just found out that for brightness controls to work the iMac cannot be locked before you go into TD mode. I tested this and works as expected.

I still think this is a bug as volume control keys still work regardless of the iMac being locked or not. Don't see why brightness shouldn't be the same.

Problem is when you go into TD mode with iMac unlocked, and your other mac sleeps, when the iMac also goes into sleep/locked state after a while you get the issue again. When that happens you need to disconnect, unlock, and connect again!

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I resolved this by unticking auto-brightness on the iMac before connecting via thunderbolt! Hope this helps

iMac Preferences

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