To see something clearly, the light coming from the object 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 lenses in our eyes, just as a camera lens has to be adjusted for near and distant objects.
A relaxed normal eye will naturally focus very distant objects. When looking at something nearby, special muscles change the shape of the lens, which is quite flexible, so that the image is focused precisely on the retina. This change in shape of the lens is called accommodation.
As we get older, our lenses lose their flexibility and ability to accommodate. Focusing on close objects becomes increasingly difficult. 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.
In short-sightedness (myopia) the relaxed lens focuses distant objects somewhere in front of the retina and they are not seen clearly. Glasses or contact lenses with special lenses which ‘diverge’ the light are needed for distant vision, often from an early age. However, when close objects are looked at the lens does not need to change its shape, which is why people with myopia often never require reading glasses.
In long-sightedness (hyperopia) the opposite occurs. Distant objects are naturally focused behind the retina. However, the eye muscles, through accommodation, alter the lens shape and glasses are often not needed initially. Close objects are also seen clearly because of increased accommodation. However, as the lens loses its flexibility this increased accommodation cannot be achieved. Thus long-sighted people will need reading glasses at a younger age (often around 30) and may eventually need glasses or contact lenses for distant vision also, as the ability of the lens to change its shape gradually declines.