Woah, wait a minute. No auto-brightness? That can't be. The specs sure indicate there is ambient light sensor. There is indeed no "auto-brightness" box to check off, unlike with other Android devices, but I thought that this new Xperia series is supposed to work like iOS devices do. In other words, you move the slider to determine the *average* brightness of the screen, just the way an iphone or ipad does. The screen gets brighter in with more ambient light and dimmer in darkness, but the average brightness is controlled by the slider.
Most other Android devices use the slider to determine a "fixed" brightness, not an average dynamic brightness.
Are you sure the brightness doesn't change?? I find that hard to believe with an ambient light sensor on board.