When a camera’s flash goes off in dim conditions, your pupils don’t shrink fast enough to block the burst of light. The result (aside from a minute of bedazzled vision): Light reflects off the red blood vessels in your retina, giving you a devilish gleam in that family photo. Fortunately, most digital cameras have a red-eye-reduction mode that pulses the flash, giving your pupils time to adjust.
When a camera flash goes off, the pupils of your subject’s eyes don’t have time to constrict to reduce the amount of light entering their eyes. Therefore, a large burst of light reaches their retinas, reflects back, and is captured on film.
Eyes look red in photos due to the rich blood supply of the choroid, a layer of connective tissue at the back of the eye that nourishes the retina and gives it its normal red color.
Picture Credit : Google
