Refractive Errors

Different people need glasses for different tasks. Some people require them to read a paper and thread a needle. Others require glasses for driving or watching television and some people require glasses for everything they want to see and do.

Hyperopia

Hyperopia, or long-sightedness, is a condition where close objects are blurry. In a relaxed hyperopic eye, the image is focused behind the retina, the back of the eye. To focus the image on the retina, a plus spectacle lens is required to be placed in front of the eye. This will allow the spectacle wearer to see all distances clearly. Hyperopia may be hereditary and often the size of the eyeball itself is slightly smaller than average.

Myopia

Myopia, or short-sightedness, is a condition where faraway objects are blurry. In a myopic eye, the image is focused in front of the retina. To focus the image on to the retina, a minus spectacle lens is required to be placed in front of the eye. This will allow the spectacle wearer to see all distances clearly. Myopia may be hereditary and often the size of the eyeball itself is slightly longer than average.

Presbyopia

Presbyopia is a common condition where the eye loses its ability to see close objects clearly. In a presbyopic eye, the lens, which would normally alter shape to adjust focus for close objects, slowly loses its flexability. This usually starts happening between the ages of 40 and 45 and can continue up to the age of 65. To focus a close image on the retina, a plus spectacle lens is required to be placed infront of the eye. This will allow the spectacle wearer to see close objects clearly, but it will make distant objects blurry. Presbyopia is experienced by everyone as it is part of the aging process and it cannot be prevented.

Astigmatism

Astigmatism is a condition where there is asymmetric blur of an object. In an astigmatic eye, one direction of the image may be focused on the retina and part in front or behind the retina. Astigmatism is usually due to the shape of the front of the eye, the cornea, not being completely spherical. This results in light which would normally be focused as a point on the retina, being focused as a line. To focus the image on the retina, a cylindrical spectacle lens is required to be placed in front of the eye. This will make objects at any distance clear.


Author: Stuart Macfarlane