Why do people have red eyes eyes in flash photographs?

Red eye reflex is seen in some flash photographs as flash photographs are taken in dull light conditions. The pupils of the subject are dilated as the ambient light is dull and the subject is looking straight at the camera. The light from the flash enters the eye through the pupil and is reflected out. This reflected light is red in colour as the retina being highly vascular is red in colour and is what is seen as the red reflex of flash photography.

This can be avoided by the subject not looking directly into the camera. A pre-flash strobe from the camera flash will cause the pupil to contract and the red eye reflex will not be seen. This is now available in most modern cameras as ‘red eye reduction’ feature.

Red eye reflex is seen more in children as they naturally have larger pupils — certain animals have a highly reflective surface under the retina giving them greater red eye reflex, that is why red eye reflex is sometimes called ‘cat eye reflex’.

Source: The Hindu