How does the auto brightness setting work ? When you turn it on, you're still able to select how bright you want your screen to be. So auto brightness would be able to brighten or dim your screen up till where you selected ?
You select a brightness level, and with auto-brightness enabled, the phone will automatically adjust to the surrounding light levels based on that benchmark level. If you do not enable auto-brightness, then the screen will stay the same, no matter how bright or dark your surroundings are.
Autobrightness will learn over time as you tweak the screen brightness for each environment. Eventually you don't have to mess with the brightness much anymore. The only exception seem to be pitch-black rooms (auto tends to go slightly too bright).
It has a mind of it's own sometimes but it can be overridden. It mainly just takes a bit to get used to.
I leave it on on my iPhone because I rarely care how bright it is.
I leave it off on my iPad because I only use 3 settings: Really bright, slightly dim (to not less distracting), and "oh #%!&, I only have 3% battery left".
I hate in the AM when I pull my phone off the clock/dock and it brightens up the whole room. I normally have my brightness at half or less. i quickly either hold it against my body to keep it a darker room or try and turn it off.