What causes red eye

A.� 'Red eye', the amateur photographers bugbear, is most common with cameras that have built-in or camera-mounted flash, where the flash unit is closest to the lens.

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This configuration means that when the subject of the photograph looks directly at the camera the flash is on the same axis as the visual axis of the camera. As the lens and flash are so close together, the light of the flash shines right through the person's iris and illuminates the retina. The retina is red in colour due the blood vessels it contains.

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So although your left with a rather demonic looking photograph, it's actually an accurate representation of what the camera 'saw', the inside of the person's eyeball.

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Q.� Does the natural colour of the person's eye have any effect on the degree of red eye

A.� Yes, people with light coloured eyes usually come out on film with the most pronounced cases of red eye. Whereas people with darker eyes may have enough pigment in their iris to mask the red eye effect.

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Q.� Why does my kittens eyes look blue in the flash

A.� Because in this case the colour reflected back onto film is that of a special reflective layer in the back of the eye, rather than the blood vessels of the retina.

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Most domestic animals have this special reflective layer, called a tapetum. Light passes through the animal's retina from outside of the eye and is then reflected back through the retina a second time from the reflective tapetal layer beneath the retina. This double helping of light helps these animals to see better than humans do in dim light situations, enhancing their nocturnal vision.

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The colour of this tapetal layer, and, therefore, the colour that you see on film, varies. Most young puppies and kittens have a blue tapetal reflection until they reach eight months and their eyes mature completely.

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The tapetal layer colour is also influenced by the colour of the animal's coat. A black coat usually displays a green tapetal reflection, whereas an off-white coat will generally show a yellow tapetal reflection.

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by Lisa Cardy