In dim light, the pupil expands to allow more light through for clearer vision. When you take a photo with a flash, pupils do not have time to contract. So the camera picks up a red reflection from retina. This is simply pigment epithelium and vascular layers supporting the retina.
Light and Dark Eyes
Red eye is more pronounced in light coloured eyes, and children with very dark eyes may have no recognizable red reflex.
Modern digital cameras have red eye reduction features that reduce or eliminate the appearance of red-eye in flash photographs. Some cameras offer both a red eye lamp or “pre-flash” and a red eye correction setting.
The Red-Eye Lamp / Pre-Flash setting causes a bright light to illuminate for about one second, milliseconds before the flash bulb lights up. Pupils contract in response to the bright light, reducing the amount of red-eye captured by the camera when the photo is taken with the flash.
Red-eye correction replaces a captured red eye with colours the camera detects in the surrounding area.
Red-eye reduction settings do not guarantee elimination of red-eye. However, the chance of the camera detecting a white pupil (the early signs of childhood cancer and other serious eye disease) are much reduced when the pupil is contracted. Therefore, taking regular photos of young children with ALL red-eye reduction settings turned OFF is very important.
If a red eye reflex is naturally absent, or a white pupil is seen in photos when red-eye reduction settings are NOT used, this could indicate a serious eye problem, such as a cataract or cancer.