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Camouflaged behind the black glass where you can't see them except with a bright flashlight are two more sensors: a proximity sensor that shuts off the screen illumination and touch sensitivity when the phone is against your head (it works only in the Phone application), and an ambient-light sensor that brightens the display when you're in sunlight and dims it in darker places.
Apple says that it experimented with having the light sensor active all the time, but it was weird to have the screen get brighter and darker all the time. So the sensor now samples the ambient light, and adjusts the brightness; it does this only once--each time you unlock the phone after waking it.
You can use that tip to your advantage. By covering up the sensor (just above the earpiece) as you unlock the phone, you force it to a low-power, dim screen-brightness setting (because the phone believes that it's in a dark room). Or by holding it up to a light as you wake it, you get full brightness. In both cases, you've saved all the taps and navigation it would have taken you to find the manual brightness slider in Settings.
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