Why do eyes glow red in flash photos? Red-eye, explained

Transcript

Why do eyes glow red in flash photos? The camera is looking straight into the back of the eye. In a dim room, your pupils open wide, up to seven or eight millimeters across, to catch more light. A flash is over in about a thousandth of a second, far too fast for them to react. So the light pours in, crosses the eye, and lands on the retina, with a layer of blood vessels behind it. Red light bounces back out. And like a bike reflector, the eye sends it almost straight back toward its source. If the flash sits right beside the lens, the camera is inside that red return beam. Now slide the flash away from the lens. Past a couple of degrees, the red glow misses the camera entirely. That's why phones, with the flash a centimeter or two from the lens, are so prone to it. Now turn up the room lights. The pupils shrink, and the glow fades. A pre-flash does the same trick: a quick burst first, so your pupils are already small for the real photo. Try it yourself at Explainer Tools dot com.