What causes redeye and how does redeye reduction work?
The interior of the human eye is lined with a fine mesh of blood vessels. If you shine a bright light into someone s eyes then this light can be reflected back and, since it s reflecting off blood vessels the light is going to be coloured red. Normally you don t notice this effect in real life for three reasons. First, the light source has to be really bright compared to the ambient lighting. Second, the pupils of the eye have to be fairly dilated for enough reflected red light to be noticeable. And third, the light source has to originate as close as possible to the viewing axis (ie: to your own eye). Unfortunately these three conditions are often met handily when you do flash photography in dim lighting conditions. Flash units produce a tremendous amount of light for a split second and, since flash units are frequently attached to the camera or built into the camera body, they re often located very close to the lens axis. The result is the evil glowing red satanic eye effect that s t
