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MYOPIA (NEARSIGHTEDNESS)
I can see close up but not far away
Myopia, also known as nearsightedness, is the most common of all refractive errors. Over 25% of the population has some degree of nearsightedness. Nearsighted people can see various ranges of near objects clearly but distant objects are out of focus. The most common cause of myopia is from an elongated eyeball. This extra length causes light from distant objects to converge to a focal point before reaching the retina. Beyond the focal point, the light rays diverge. Since the retina only captures the quality of the image that reaches it, the brain receives an image that is out of focus.
HYPEROPIA (FARSIGHTEDNESS)
I can see far away better than close up
Hyperopia, also known as farsightedness, is a refractive problem caused by an eyeball that is too short or a cornea that is too flat. When the eyeball is too short, the reduced length does not give the cornea and lens sufficient space to bring the light rays to a focal point upon the retina. When the cornea is too flat, it does not bend the light rays from near objects sufficiently to bring them in focus upon the retina.
ASTIGMATISM
Astigmatism is an overall inability of the eye to focus clearly at any distance because of uneven curvatures of the cornea. Instead of having uniform curvatures in all meridians, astigmatic corneas have more curvature in one meridian than the others. Corneas with pronounced astigmatism are shaped more like a football than a well-rounded basketball.
Every cornea has some degree of astigmatism. Fortunately, for many individuals, the distortion due to uneven curvature is so slight that it has little impact upon vision. However, as the degree of astigmatism increases, the level of distortion increases proportionately. Astigmatism often occurs in conjunction with nearsightedness and farsightedness.
PRESBYOPIA
I am over forty and cannot see close up any more
In young individuals, the natural lens of the eye is soft and pliable. This innate flexibility permits the natural lens to change its shape, allowing it to focus on objects near the eye. As the years pass, the lens loses its flexibility and can no longer vary the focus of the eye. This condition usually becomes noticeable sometime between 40 and 50 years of age. People with normal vision up to that time find it increasingly difficult to focus on near objects, like words on a page or a computer screen, and need to wear glasses for reading and other close up activities. |
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