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EndlessMac

 
Member Since: Jan 18, 2010
Posts: 1,469
EndlessMac is a jewel in the roughEndlessMac is a jewel in the roughEndlessMac is a jewel in the rough
Mac Specs: 2.8 GHz 15" MacBook Pro OS X 10.7.x & some old Macs

EndlessMac is offline
I don't know if the iMac has the same ambient light auto adjust as the MBPs do but if it does then that's likely the causes of your problem. To fix it go into system preferences and in the display icon uncheck the automatically adjust brightness as ambient light changes.
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