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The Secret Behind Red Eyes in Old Pictures! The red-eye effect in photography is caused by the reflection of a camera flash off the blood vessels at the back of the human eye. When a flash occurs in low-light conditions, the pupils are dilated to allow more light in. This leaves the retina exposed. The light from the flash enters the eye and reflects directly back through the pupil, particularly when the flash is very close to the camera lens. Since the choroid layer of the retina is dense with