Why do eyes look red in photos?

In dark rooms our pupils widen to let in more light. A camera flash fires far faster than the pupil can shrink, so the light passes through, reflects off the choroid, a layer full of blood vessels behind the retina, and returns to the camera as a red glow. The effect is strongest when the flash sits close to the lens, as on compact cameras and phones. Red-eye reduction modes fire a short pre-flash so the pupils contract before the photo is taken. Animals such as cats show green or yellow eyeshine instead because of a reflective layer called the tapetum lucidum.