how does auto brightness work? there is a similar question to this one How does the automatic brightness settings work?

however, I do not think it answers my question. This is more to do with the UI.

I just got myself a Galaxy s3. The phone has under the notifications the ability to change the brightness.


As can be seen from the screenshot, I can both adjust the brightness manually and select auto. Adjusting the brightness manually does visually affect the brightness of the screen even when the autobrightness is checked.

So what does the manual adjuster do when auto-brightness is turned on?
The Auto option enables the lightsensor of the device.
On some Samsung devices there remains a slider when the checkbox Auto is checked. With that slider you can set a delta (small change) in the brightness set by the data of the sensor. You can chose if you want it a bit brighter or less bright than the sensors standard value.

In short:
 - Box checked: Sensor defines brightness (+/- delta)
 - Silider in the middle: delta = 0
 - Slider to the left: delta = - amount of steps from the middle
 - Slider to the right: delta = + amount of steps from the middle