The red-eye phenomenon reveals itself when a flash close to the lens axis is reflected back to the camera by the subject's eyes. The reflected light is turned red or pink by blood vessels in the retina at the back of the eye. The effect varies from person to person, but is at its worst in poor light when the subject looks directly into the lens. It is most likely to occur when the angle subtended by the flashgun and lens, as seen by the subject, is less than about two degrees. It can be reduced or eliminated by averting the subject's eyes, moving closer to the subject, shifting the flashgun away from the lens axis, or using a red-eye reduction lamp to contract the iris and reduce the diameter of the pupil prior to releasing the shutter.
The red-eye reduction facility found on some flashguns emits a series of low-level pre-flashes prior to the release of the main flash. These additional flashes occur before the camera's shutter is opened and are designed to encourage the iris of the eye to close down to a smaller diameter. This reduces the chance of red-eye occurring but cannot completely eliminate the phenomenon.