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When I set my brightness on a 27'' iMac (2011) next time it boots up, it's dimmed. I switched off the ambient sensor, so this shouldn't be the problem. The connected Cinema Display remembers the brightness correctly.

Had anyone the same problem and found a solution?

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What version of OS X are you running, and which brightness level do you have it set on? – l'L'l Oct 9 '11 at 18:32
Snow Leopard with all updates applied. For testing purposes I have tried to set it to maximum, but it returns to three or four ( I must reboot to look for the exact level ) – Sascha Oct 9 '11 at 19:34
The exact level it reverts to is three. If I remove the two Cinema Displays attached, it reverts to six. The Cinema Displays aren't affected by this behavior. – Sascha Oct 9 '11 at 19:40
Try setting it one or two levels below maximum and restart. I know that people have said if it's set to the max or min level that can happen. If you still have the same result there's a couple more things you can try. I could make a tiny script that will automatically boost it up where you want it whenever you restart, etc. if you run out of possible possibilities. – l'L'l Oct 10 '11 at 0:23
Tried to set it to a value lower than max and rebooted. Reverted back. I'm going to test that tomorrow with the Cinemadisplays disconnected. – Sascha Oct 10 '11 at 20:05

I have heard a few people complain of brightness resetting on reboot. Traditionally brightness settings are stored in the PRAM, which you can reset via PR on booting. I would be interested to see if it gets "stuck" on a brightness setting that you do not wish to have after resetting the PRAM.

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Does not changed the behavior. Rebooted and hold down the key combination ( it rebooted again ). Then set my desired brightness, rebooted and the same problem again. – Sascha Oct 9 '11 at 14:56

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