It works the same from what I can see.
What I noticed the other day (which is how I would expect auto brightness to work). I was setting my iPhone 5 up. I was sitting with the sun shining through the windows (which was directly behind me). I had set the brightness level to around 75 percent, I had auto brightness on. When I went back into the brightness section again, due to the bright sunlight, I saw the brightness level move its self up to the maximum level. It done this because of the bright sunlight and so that it would be easier to read. This is exactly how I would expect auto brightness level to work, i.e dim itself when in a darkened room, and brighten up when in a well lit area.
If you go into the section to alter the brightness, and set it at say 75 percent, put auto brightness on. Leave the screen on for the brightness section and go outside in the sunlight. You should see fairly quickly the brightness level go up all by its self. I forgot to mention that both my iPhone 5 and iPad 3 do this.
Refer to Figure 1 in http://www.displaymate.com/AutoBrightness_Controls_2.htm
Prior to iOS 6, the control function of Auto Brightness is pre-determined based on the Brightness slider value you set. Figure 1 shows 4 such curves; 1/4, 1/2, 3/4 and Max.
In iOS 6, http://www.reddit.com/r/apple/comments/10a9p0/til_how_the_ios_6_auto_brightness_ works/ propose that users can dictate their own brightness function.
- In ambient brightness of value a1, if you're not happy with the proposed brightness by the phone, you set it to some brightness level, say b1.
- In ambient brightness of value a2, if you're not happy with the proposed brightness by the phone, you set it to some brightness level, say b2.
- In ambient brightness of value a3, if you're not happy with the proposed brightness by the phone, you set it to some brightness level, say b3.
From now on, each time the phone goes into ambient lighting of level a2, it will automatically go into brightness level b2.