11 Replies Latest reply: Jan 8, 2016 12:08 AM by lotsofmacstuff
citan Level 1 (0 points)


i'm using my iMac 27" as external monitor for MacBookPro (via minidisplayport cable).

Recently i noticed that i can't change brightness on iMac (when used as external monitor). Previously i was using iMac keyboard to adjust volume and brightess even when connected to MBP.


I'm sure it was working before. I suspect 10.6.8 update broke something.

Can anyone confirm? Do you have a solution?

Full brightness makes iMac screen unusable for me. :/




iMac, Mac OS X (10.6.8)
  • Fortuny Level 7 (21,435 points)



    maybe you changed the behavior of the Function-keys accidentally.


    Have you tried the keys with the 'Fn'-key ?







  • citan Level 1 (0 points)

    Hi Stefan,

    i should write it in the first post. Brightness controls work fine if iMac is not connected to MacBook, do not work when connected to MBP.




  • Fortuny Level 7 (21,435 points)

    Maybe a SMC-reset http://support.apple.com/kb/HT3964 can help


    Haven't had such problem myself yet.

  • citan Level 1 (0 points)

    Ah, i forgot to write it in the first post. I did it, didn't help.

  • Fortuny Level 7 (21,435 points)

    You don't seem to be alone with it https://discussions.apple.com/message/15515094

  • 36Joules Level 1 (0 points)

    I posted this in the linked discussion thread above, but just in case it might help someone finding this thread:


    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


    This has worked consistently for me. Hope it works for you, too. Hope Apple fixes this permanently somehow soon, it sure is annoying.

  • tgines Level 1 (0 points)

    If you have a wired keyboard, plug that into the iMac and use the brightness keys to get your desired brightness. You can do this while the iMac is the second monitor. Worked for me.

  • zead29 Level 1 (0 points)

    I had the same problem with 10.6.8. But it works well with the mountion lion upgrade.

  • datsolalee Level 1 (0 points)

    DUDE - you're a genius!  Thanks for this, I've been squinting for the last month because of my bright iMac (aspecially at night) and I'm on deadline so HAVE to work all night...


    Thank you thank you thank you!!!!

  • MediaRaver Level 1 (0 points)

    ONE MORE SUCCCESSFULL SOLUTION. I have iMac 27" with Mini DisplayPort used as an external monitor for Macbook Pro Retina 15" with Thunderbolt port connected with MDP cable. After the iMac system was automatically updated to version 10.6.8, the brightness stopped adjusting by pressing F1 and F2 keys on the wireless iMac keyboard during the connection. I've tried to reboot iMac in different special modes, reseting, etc..., but all the attempts failed. Finally I've remembered that I still have the original istallation disc with the older OS X version, that was provided with the iMac. I've used it to install OS X 10.6.4, and got all the brightness functions back, constatly working after each reboot. Of course, it would be better to have the same functions in the latest OS version, but now I'm quite happy to get the external monitor mode properly available again.

  • lotsofmacstuff Level 1 (0 points)

    Just in case people end up here looking for a solution for adjusting the brightness of an external monitor or iMac when connected to a macbook pro.  I had the same issue as everyone in this forum.  I have a late 2015 Macbook pro and I wanted to use my late 2009 i7 iMac as my monitor.


    First issue

    1. Don't use a Thunderbolt cable to connect the old iMac's display port.  The only thing that works is a display port cable with dual male ends.  This will work.

    2. The whole F2 thing does not work so don't waste your time.


    Second issue:

    1. When I use my iMac as an external/additional display/monitor for my new Macbook Pro I can't control the brightness of my iMacs screen through the Macbook pro.

    2. Instead of experimenting save time and go to the App store and purchase a program called ScreenDimmer.

    3. Install this app and a dark screen icon will be added to your menu bar at the top right of your screen.

    4. Click on it's icon and it will allow you to adjust the brightness of 3 separate, connected displays/monitors.

    5. This will work trust me.