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 by 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.
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.
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.
Please see LASIK for details on specific techniques.