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Intro
Approximately one in four Americans is nearsighted, totaling 70
million people. The degree of myopia can vary from low (-1 to -3
diopters) to high (greater than 6 diopters). Myopia is not caused
by reading at an early age, prolonged reading, reading in the dark,
watching TV too closely, wearing glasses too strong, wearing glasses
too weak, or other similar folklore. Some controversy exists over
whether myopia can be made worse be prolonged near work as a child,
but this has never been proven.
In studies, it has been correlated with higher I.Q.s., but this
probably has to do with the fact that myopes tend to read more since
that's where they can see the best without glasses (see below).
Myopia tends to run in families, so it is probably inherited. Myopia
tends to start in the early teens (earlier for high myopes), and
increases as the eye grows in length during puberty. It tends to
stabilize after age 18.
Optics
 In
a nearsighted eye, the cornea is too steeply curved for the length
of the eye, causing light rays to focus in front of the retina.
Distant objects appear blurred or fuzzy because the light rays are
not in focus by the time they reach the retina. In the picture above,
the arrow shows the focus of the nearsighted eye while the dotted
lines represent the normal eye. The greater the myopia, the more
the light rays converge and the more blurred distant objects are.
Near objects, viewed at the proper distance, can be seen clearly
because the focus of their light rays matches the refractive error
of the nearsighted eye. For example, a patient with -2 diopter correction
is naturally in focus at 0.5 meters (18 inches). Having the focus
point of the eye within arm's length is an intrinsic advantage to
the myope, who always has the option of seeing well at near without
optical correction. Indeed, many myopes instinctively remove their
spectacles for near work, particularly after the onset of presbyopia.
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Spectacle Correction
The correction of myopia with spectacle lenses accrues advantages
and disadvantages for the myope. The obvious advantage is bringing
distant objects into focus. The disadvantages increase in rough
proportion to the strength of the lens. Most myopes begin wearing
spectacles in childhood and therefore incorporate the less desirable
cosmetic, convenience, and optical disadvantages into their daily
routine; however, this adaption varies among individuals.
Among the optical drawbacks of minus spectacles lenses is the minification
of the image. There is a 2% minification for every diopter of spectacle
power. For example, a -10 diopter spectacle correction results in
a 20% reduced image size. Thick lens edges and supporting frames
also distort and reduce the peripheral vision.
Contact Lens Correction
Contact lens correction reduces the optical problems that plague
spectacle lenses, but they have their own unique problems including
high-maintenance care solutions, corneal warpage, corneal suffocation,
corneal infections, and eyelid allergies. Some people lose their
ability to wear contact lenses - particularly individuals with dry
or sensitive eyes.
Surgical Correction
Please see LASIK for details on specific techniques.
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