The consensus view seems to be that using Auto Brightness shortens battery life. And that seems to be consistent with my own experience as well. What is it about that feature that most significantly contributes to compromised battery life? Is it the fact that it will allow the device to go to max brightness in relatively bright surroundings? Or does it have more to do with the fact that the ambient light sensor is constantly working? If the former, it would be nice if iOS allowed you to specify a maximum (and a minimum) brightness setting that the Auto Brightness feature would stay between.