iPhone auto brightness

This is a great feature and althouh many people say it seems to do nothing I think that it works so well that it is unobtrusively cool.

The feature takes a reading of ambient light when the phone wakes from sleep but it also periodically adjusts brightness when you go say from a dark room to a well lit one.

You can see the effect like this.

Sleep the phone and do to a dimly lit place. Wake the phone and see the screen brightness. If you then turn on a bright light or move to a mote brightly lit room then you will see the brightness increase to suit the current conditions.

The phone doesn't seem to be so keen to reduce brightness when awake but it always adjusts on waking.