Why do people have red eyes eyes in flash photographs?

R. Vinith Gandhi
Tamil Nadu

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’

Dr. Arumozhi Varman
Uma Eye Clinic, Anna Nagar Chennai

The red eye effect is common in most photographs when the flash is taken too-close to the lens. Red eyes are caused when the light of the flash occurs too fast for the iris to make the pupil of the eye close. The light of the flash is focused on to the blood rich retina at the back of the eye and the image of the illuminated retina is transmitted to the camera resulting in a red eye effect.

The effect is generally more pronounced in people with grey or blue eyes and in children. This is because the pale pupils have less melanin in them and so allow more light to pass through to the retina.

Children, despite superficial appearances, do not have larger pupils but their pupils are more reactive to light and are able to open to the fullest extent in low light conditions. Many adults lose the ability to fully open their pupils except through the use of drugs.

Arun Radhakrishnan
Thrissur, Kerala