How does iPhone's automatic backlight adjustment feature work?

[illjazz]

I'm a bit puzzled by it. I have my brightness at a mere 20% or so because it's totally sufficient and still looks very nice, and I have automatic backlight adjustment turned on. If I jack the brightness up to 80% or 100%, still with automatic backlight turned on, it seems to just stay at that percentage rather than automatically lowering the brightness. What's the deal with this feature? How does it work?

I never notice it making the screen dimmer or brighter based on the surrounding lighting conditions.

[jasunto]

i keep mine really dim and notice if under my shirt the screen is dark at first, what i want to know is exactly where the ambient light sensor is located on the iphone

[pcuserforever]

gotta be by the earpiece, thats why the screen shuts off when you put your ear next to it in a call........

[illjazz]

gotta be by the earpiece, thats why the screen shuts off when you put your ear next to it in a call........

It's easy to test as well. Just put your thumb over the ear piece, roughly, and the screen will dim just like when you put it up to your face.

I just don't quite understand the logic. In fact, when the automatic backlight feature is enabled, the bar for brightness adjustment should be unavailable to the user, because, well, it's automatically adjusted! That's the whole point of the feature. Instead, iPhone, for some strange reason, still lets you use the slider even with automatic backlight turned on. I don't get it.. hence this thread and the question about how it works.