Red-Eye in Photos is a Reflection of Your Blood
When the flash of a camera goes off, the eye isn’t prepared for the sudden influx of light, and the pupil doesn’t have time to restrict. You’re likely using flash in dark lighting, so your eyes have dilated to adjust to the dark room.
When the flash goes off and the photo is taken, your eyes are still dilated, so the light reflect off of the red blood vessels of the choroid, which is the layer of connective tissue in the back of the eye that nourished the retina.
