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I have a 2 week old 24" iMac running Mac OS X 10.6. It is the primary monitor with the menu bar on the top of display. I have an external monitor as well, a 19" Viewsonic LCD. The LCD is set to the left side of the iMac, rotated 90 degrees CCW and has the Dock along the far left edge.

When I restart the screen brightness on the iMac reverts to Full brightness. The Viewsonic LCD retains the setting I have for it. I am using a Mini DVI to DVI cable for the external display.

I even tried setting my Huey Pro to do automatic screen adjustment based on ambient lighting but the iMac still goes to stun with a reboot.

I am sure it is something dumb I have overlooked.

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up vote 4 down vote accepted

For clarification, do you set the brightness to the lowest possible? It appears there is a bug in 10.6 that won't save your brightness if it is set to the lowest possible. Any other notch and it saves. My iMac has personally never exhibited the issue but I never use brightness down all the way - I'm normally at half.

Sources: Apple Discussions, ehMac

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Never would have guessed that in a million years. But yes, that is exactly what I was doing. Mad props for digging that esoteric bug out of the Apple DB. – TomB Sep 13 '09 at 1:47
That's what SuperUser should be for :-) – Chealion Sep 13 '09 at 3:58
Seems like this bug has crawled into 10.7 as well. I came here looking for just that. – aalaap Jul 8 '11 at 9:47

all macs (iMac, MacBook or MacBook Pro) have ambient light sensors which adjust monitor brightness to the environment.

If you dim the lights in your room or around your iMac - you should see that the screen brightness is reduced too. You can turn off "auto-brightness" settings in Display preferences.

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That is true for all Mac Laptops but neither of the two iMacs I have seem to have an ambient light sensor. Nor is there an "auto-brightness" setting in the Display Preferences. – TomB Sep 12 '09 at 21:45

I use darkadapted to set a brightness for my Cinema Display monitors. It lets me set a brightness and save the settings. I find the results better than using Apple's brightness settings. Darkadapted is trialware but I think there are free programs that perform the same functions.

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To my knowledge it is not true that iMacs have brightness sensors. Maybe the new 2009 models do, but the others don't.

I can confirm the issue that the brightness value does not get restored upon restart on my iMac 2007 model. However I have not tested Chealion's suggestion that only the lowest nothch is affected.

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this is a known bug and the only solution is to turn brighness not to 0 (zero) but 1. then it will be saved. it is not only on 10.6 biut on 10.5 too, seems to be a hardware design flaw on newer Macs …

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