שךום רב! Cause of red eye If you use a flash in a dark environment, you often get a red eye effect. This is because the light of the flash is reflecting from the retina, which is covered with tiny blood vessels. The more open the pupils are, the more red eye effect you get in your photos. Red eye is more pronounced in people with light eye color. It is also more pronounced in people with blond or light-red hair and in children.