Red eye is a common photographic effect that occurs when light from a flash unit reflects off the blood-rich retina at the back of a subjects eye and travels directly back along the lens axis during exposure. This results in the subjects pupils appearing bright red in the photograph.
Red eye is most frequently seen in low-light or indoor photography, where the subjects pupils are dilated. Photographers can reduce or eliminate red eye by using off-camera flash, bouncing the flash, red-eye reduction pre-flashes, or post-processing software to correct the effect, producing more natural-looking portraits.