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What causes red eyes when you take a picture of someone?

There is a reflection of the flash off of the back of the retina - the blood vessels in the retina make the red light seem predominant.

Ellen Mayo

To prevent this, fancy new cameras have an anti-red-eye feature. The flash strobes at low power for a second before the picture is actually taken. The light causes the subject's pupils to constrict, so that when the picture is taken you don't see much of this reflection.


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