I am sure auto brightness would save battery life but if you compare the low setting to auto brightness most of the time auto brightness is darker then low. Just set you phone to low and switch auto brightness on and off and you'll see. I don't know how it could get lower then low...but its annoying. My wife's Focus S doesn't do it and my old Titan didn't either. Go into a somewhat low lit room or just cover the proximity sensor to test.