Rebecca Large has always been told her big brown eyes are her best feature. Like those of Hollywood stars Angelina Jolie and Audrey Hepburn, Rebecca's eyes are regarded by most as a sign of beauty.

 

Yet eye experts have realised that far from being an asset, large eyes can actually be a weakness, and make someone more prone¹ to being short-sighted.

 

This condition, also known as myopia, causes distant objects to appear blurred², while close ones can be seen clearly. It is caused by light not properly reaching the retina（视网膜）, the light- sensitive area at the back of the eye.

 

Experts believe this might be because the eyeball grows too large, which causes the light to focus in front of the retina, rather than on it.

 

Short-sightedness is on the rise in Britain, with up to 50 per cent of people now estimated to have it.

 

One possible explanation for this is that our eyeballs are getting larger because we are spending more time indoors. A lack of outdoor light in our early years causes the size of the eyeball to increase. But the larger the eyeball, the more likely a person is to be short-sighted, explains Andy Luff, a consultant³ ophthalmic（眼睛的） surgeon at Optegra, a chain of private eye hospitals.

 

'While big eyes can certainly look beautiful, the fact is that if your eyes are slightly too big, this will cause you to suffer short sight. If the eyeball is too big, the light focused by the lens of the eye does not reach the back the retina, where images are processed.

 

'If the eye is only 1 mm longer than 24 mm (the average size from front to back) a person will be significantly short-sighted. Longer than the average by 2 to 5mm will make you highly short-sighted.'

 

Dr Luff explains that in the past we thought the eye grew too large and became short-sighted due to genetic⁴ factors.

 

'Although genetics is still the most important reason behind short-sightedness, lack of exposure to outdoor light is another possible cause. And it's the one preventable cause which can be addressed in childhood.'

 

Our eyes usually reach their full size by our 20s. The ideal is that they stop growing at the right size and will have perfect vision, but if they grow too big they'll be short-sighted and if they don't grow enough they're long-sighted, and won't be able to see things close up. Some children can have eyes that are too large by the age of five, says Dr Luff.

 

A review published by the University of Cambridge in 2011 found that for each additional hour spent outside each week, the risk of short-sightedness was reduced by 2 per cent. The review of studies involving 10,000 children and adolescents found that short-sighted children spent on average 3.7 fewer hours per week outdoors than those with normal vision or long-sightedness.

 

But how could exposure to light change the shape of the eyeball? Dr Saunders suggests that natural light in the eyeball flicks⁶ a type of biological switch that prevents it from growing too large. However, artificial light is not strong enough to flick⁵ this switch.

 

People with small eyes tend to be long-sighted and will have difficulty seeing close objects clearly because light rays are focused behind the retina.

 

But this tends to be largely genetic. 'Too much outdoor light will not make your eye long-sighted,' says Mr Luff. He adds that short-sighted people seem to suffer more complications than those who are long-sighted.

 

'We see far more associated eye health problems in the short-sighted than the long-sighted, so it's worth doing everything you can to try and prevent it,' he explains.

 

However, Dr Susan Blakeney, clinical adviser⁷ to the College of Optometrists⁸, says there could be other factors contributing to why our increasingly indoor lifestyles may be causing bad eyesight.

 

'We still don't know what it is about time spent outside that has a protective effect against short-sightedness - it could be the sunshine, the brightness of the light or the fact that objects you focus on tend to be farther away outdoors.'

 

In rare cases short-sightedness can also be caused by the curve of the cornea（角膜） (the transparent⁹ part of the front of the eyeball) being too steep.

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