Do you know what causes "red eye" when you take a flash photograph?
The choroid is a layer of tissue at the back of the eye that contains a large number of blood vessels. Red eye usually happens when a flash photograph is taken in dim light. In dim light, the pupil of the eye is dilated and allows plenty of light to enter the eye. Red eye is caused when the choroid reflects the light of the flash. The pupil does not constrict fast enough to reduce the amount of light that enters the eye. Therefore, the flash of light reflects back out of the eye and is recorded on film. Some cameras use red eye reduction methods that send out a short burst of light before the film is exposed. The brief burst of light allows the pupil to constrict and thus reduces red eye.
