Ambient light sensor not working when moving from light to dark rooms with an iPhone or iPad Over the years, I've collected a few iPhone 4's and 4S's, as well as a couple iPad 2's and a new/third generation iPad. They're all equipped with an ambient light sensor that darkens the display in dark settings or brightens it in bright settings. All in all, this has proven to be a boon to me over the years, and made my experiences with the iOS family of devices a generally positive one.

However, I've noticed a consistent problem that still has yet to be addressed on any of them, and it seems like it wouldn't really be much of an issue for Apple to fix: Though it works without a problem (and very quickly) to adapt to the changes from a dark to a light room, none of my newer iOS devices will dim when moving back to a dark room; regardless of how much time I give them. The only fix to this particular issue being to shut off the screen, then turn it back on, which instantly sets it to its auto-dimmed state.

I won't accept that I've somehow managed to accrue a collection of the only iOS devices that have faulty ambient light sensors, and I know that it's not a fault of all of Apple's devices with the sensors (because my iMac, MacBook Air, and MacBook Pro will auto-brighten and dim all day long as the sun shines and then gets covered by clouds, only to emerge triumphant a few minutes later).

So what gives? Is there a fix that I'm missing? Is this a "feature"? If not, why hasn't Apple bothered to fix it?

For the record, this has been on every increment of iOS since 4, (unfortunately) including iOS 6 on the devices I use for development.
I have noticed that Auto-Brightness works much better on my iPhone then it does my iPad 2, but here are a few things you can check...

Go to settings > Brightness &amp; Wallpaper 

Slide the brightness slider to the left... Doesn't really matter where as long as it isn't all the way to the right (max brightness) mine is about in the middle, make sure the toggle for Auto-Brightness is set to on.

You can test it by:

Press the "Home" button and then lock your iPhone by pressing the "Sleep/Wake" button. Move into an area with bright ambient light and use your hand to cover the top third of your iPhone screen. Press the "Home" button and move the slider to unlock your phone. The screen should appear dimmed. Remove your hand from the top third of the display. If auto-brightness is working properly, your screen will get brighter.

You can read more about it here: http://techtips.salon.com/iphone-auto-brightness-works-3626.html