Red Eye Blues

We are all familiar the strange effect produced by the flash of a camera but what is it and, more importantly, how can we avoid it?

If you catch an animal in headlights you often see their eyes glowing mysteriously green, but the same effect doesn't happen with human eyes. Many animals have adapted to low light conditions and have an extra layer in their eye to help them survive in the dark and this layer has a mirror like surface ( called the tapetum lucidum ) which is what we see refected. In the human eye we simply see a reflection from the retina: the light-sensitive surface at the back of the eye that is effused with blood vessels, so the reflection is red.
How much of the reflection we see is slightly more complicated, but in low light conditions our pupils are dilated (bigger) and this allows more light to travel in and out, and more light = brighter reflection.

 

 
dilated pupil diagram
dilated pupil digram

A dilated pupil allows more light through and illuminates a bigger area of the retina. Our pupils become dilated in dark or low light conditons

The reverse effect in bright conditions: less light going in and a smaller area lit up.

Typically it is photographs taken at a party with a smart phone or small compact camera, that most often cause problems.

The problem with small cameras is that the flash is close to the lens and, like a snooker ball hitting the cushion at right angles, the light goes straiight into eye and bounces straight back out. If the light went in at an angle the reflected light is deflected and is just adsorbed inside the eye. Professional flashguns are positioned several inches away from the camera body and this increases the angle enough to solve the problem.

Camera manufacturers have adoped a couple of methods to overcome the effect by making the pupil contract either blasting a continous bright light in the face of the subject or firing one or two small pre-flashes, making them blink and tense up. Worse still there is always a significant lag between the 'perfect moment' and when the camera finally decides to fire the shutter. This solution is a compromise and not a very good one.

 

 
no polariser
with polariser

The built-in flash of a small camera is too close to the camera lens and the reflected light bounces straight back from the eye.

With a greater distance from the lens, a professional flash increases the angle going in and so the reflection is just adsorbed in the eye.

The problem with small cameras is that the flash is close to the lens and, like a snooker ball hitting the cushion at right angles, the light goes straiight into eye and bounces straight back out. If the light went in at an angle the reflected light is deflected and is just adsorbed inside the eye. Professional flashguns are positioned several inches away from the camera body and this increases the angle enough to stop most red eye.

 

   
smart phone
digitalcompact camera
pro camera and flash

The lens and flash are within a few millimeters on a smart phone

The flash on a compact camera is still too close to the lens The professional set up avoids red eye by placing the flash away from the lens

What if you already have photographs with red eye?
The science behind the causes of red eye may be useful information but how can the effect be removed from images. Once again manufacturers of cameras and software offers various solutions and it is still possible to buy a felt-tip pen. Marketed as a 'specialist red-eye removal device' , it is simply a felt-tip pen, but if you feel competent enough to place a blob of ink on a print without making the person cross-eyed, then go ahead!

A more satisfactory, though more complicated, technique is to use a software package such as Photoshop. Enlarge the image sufficiently to work comfortably and use either a pen-tool or colour selection to isolate, then cover over the red area. It is something that requires patience and practise but the joy of modern software means that you can use layers and history so that if you make a mistake it is easy to go back and try again. Always use a copy of the original file too, just in case.

If you're pushed for time or still stumped by the technology please give us a call: we can make the necessary changes with only a small charge and if it means saving a special photograph then the improvement is worth it.


Alistair Baird
The Darkroom UK Ltd
www.the-darkroom.co.uk
01242 239031