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The eye functions very much like a camera. Light enters the eye through the cornea, passes through the pupil, and is focused by the lens onto the retina. For images to be perceived clearly, the light rays must be focused properly on the retina otherwise, a blurred or "fuzzy" image is seen. This condition wherein the images are not properly focused is called an Error of Refraction (EOR). In general, EORs may cause a person to be nearsighted, farsighted, or astigmatic
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Myopia |
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Myopia is the medical term for "nearsightedness", a condition wherein the eyes can see close objects but are unable to see distant objects clearly. Compared to the normal eye, a myopic eye is longer from front to back. As a result, light is focused in front of the retina, rather than on the retina, resulting in the perception of a blurred image.
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Astigmatism |
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In order to have normal, undistorted vision, the cornea should be smooth and spherical or equally curved in all meridians. Some eyes have some degree of astigmatism, or ovalness, to their cornea. Instead of being shaped like a basketball, the astigmatic eye is shaped more like an American football. Therefore, there is distortion or tilting of the images due to asymmetric bending of the light rays. This is sometimes perceived by the individual as a "shadow effect" or a doubling of the image.
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Hyperopia |
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Hyperopia is the medical term for "far-sightedness." It is a misnomer, however, in that people do not necessarily see clearly only for far (distance). As opposed to myopia, a hyperopic eye is shorter when compared to a normal eye. This results in light being focused behind the retina causing the perception of a blurred image.
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Presbyopia |
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Presbyopia is not an EOR, but rather, an age-related loss of the focusing power for near vision. It usually sets in as one approaches the age of 40 or thereabouts. Normal and farsighted individuals usually require reading glasses once they reach this age. A presbyopic person with a moderate amount of nearsightedness may need only to remove his corrective spectacles for near work or reading.
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