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Why do people require glasses?

To see something clearly, the light coming from the subject we are looking at has to be focused on the retina at the back of the eye. This focusing is done by changes in the lens of our eye, just as a camera lens has to be adjusted for near and distant objects.

A relaxed normal eye will naturaly focus very distant objects. When we look at something nearby, special muscles change the shape of the lens, which is quite flexible, so that the image is again focused precisely on the retina.

This change in shape of the lens is called accomodation.

As we get older, our lens loses it's flexibility and ability to accomodate. Focusing on close objects becomes increasingly dificult. This is why many people with 'normal' eyes will need reading glasses some time after the age of 40. This is a natural process called presbyopia.

Short - Sightedness

In short-sightedness (Myopia) the relaxed lens focuses distant objects somewhere in front of the retina and they are not seen clearly. Glasses with special lenses which 'diverge' the light are needed for distant vision from an early age. However, when close objects are looked at the lens does not need to change it's shape, which is why people with Myopia often never require reading glasses.

Long - Sightedness

In long-sightedness (hypermetropia) the opposite occurs. Distant objects are naturally focused behind the retina. However, the eye muscles, through accomodation, alter the lens shape and glasses are not needed initially. Close objects are also seen clearly because of increased accomodation. However, as the lens loses it's flexibility this increased accomodation cannot be achieved. Thus long-sighted people will need reading glasses at a younger age (often around 30) and may eventually need glasses for distant vision also, as the ability of the lens to change its shape gradually declines.

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