Refractive errors of the eye

When light strikes the surface of the eye, it is focused by the cornea and the lens onto the retina, which is the sensing tissue that relays the message via the optic nerve to the brain (the brain really does the "seeing"). Slight abnormalities in this focusing mechanism may cause patients to be nearsighted, farsighted, or to have astigmatism. With age comes a decrease in the eye’s ability to focus on near objects; this is called PRESBYOPIA. Astigmatism and presbyopia are dealt with in separate summaries.

Nearsighted patients have too much focusing ability in their eyes and therefore require MINUS lenses that subtract power from the eyes and bring distant images into sharp focus. These lenses also create a slight minification of the object and tend to be thick on the outside. Near vision in these patients is excellent without glasses. Sometimes the wearing of their distance glasses renders reading vision more difficult. The other name for nearsightedness is MYOPIA. The first symptom of myopia is blurred distance vision, often with night-time activities or driving. 

Farsighted patients have less power in the focusing mechanism than is desirable and, therefore, require additional PLUS power. This can be obtained from two sources: The first is from the eye's own natural, dynamic focusing mechanism within the lens of the eye.  In childhood, large amounts of farsightedness can be overcome naturally. However, as we age we lose this ability and we become dependent on the second source, which are glasses.

It is very common for infants to be farsighted and then experience a decrease in their farsightedness as they go through childhood. In mature adults farsightedness tends to increase again. The medical term for farsightedness is HYPEROPIA. Corrective lenses for hyperopia tend to be thick in the center and create a slight magnification of the object being seen. The first symptom of hyperopia is blurred vision with close-up activities, like reading and sewing. Often headaches and eye-strain are reported by hyperopic patients.

Myopia, hyperopia and astigmatism can all be considered slight errors from perfect vision. Happily, modern science and technology can easily correct these with either glasses or contact lenses.