How much does auto-brightness drain the battery?

Specifically on the iPhone, but other phones as well. Auto brightness uses the light sensor to adjust the brightness constantly. How much extra battery does that use throughout the day?
Can I save battery by manually adjusting the brightness when needed (or leaving it low)?

You will save the most battery life if you just leave it at the lowest setting and manually adjust it on your own. Sometimes the auto light sensor doesn't dim the phone enough where you can still see it, so it's best to do it manually. The screen is the biggest drainer of the battery, so by keeping it at the lowest setting and manually adjusting, you'll save the most battery power.