Red eye reduction is a camera feature or technique designed to minimize the red-eye effect caused when a flash reflects off the blood-rich retina of the eye. This effect is most pronounced when the flash is close to the lens axis, as with built-in camera flashes.
Red eye reduction works by firing a pre-flash or series of pre-flashes before the main exposure, causing the subjects pupils to constrict. Smaller pupils reflect less light back to the lens, reducing the intensity of the red-eye effect. While it does not eliminate red eye completely, this method makes the effect less noticeable, and it is commonly used in portrait and event photography where subjects are close to the camera.