Focusing errors keep you from seeing objects clearly and may be hereditary or may be caused by disease, age, or injury.
In normal 20/20 vision, the lens of the eye focuses an image of its surroundings onto a light-sensitive membrane in the back of the eye, called the retina. A round cornea allows the focus point to be directly on the retina thereby producing a clear image.
Perhaps the most common focusing error in the United States is myopia, or nearsightedness, with over 1/3 of the population experiencing this visual problem. With myopia, either the eye is too long, or the cornea is too steep, causing light rays to come into focus before they reach the retina. This causes objects at a distance to be blurry, while close objects are seen clearly.
There are varying degrees of nearsightedness which are determined in diopters. The more nearsighted you are, the more dependent you will be on glasses or contacts to see objects at a distance clearly. A common determinant for the need of glasses or contacts can be obtaining a driver's license. Children who are nearsighted are usually discovered in grade school when they are unable to see the front of the classroom or complain of headaches while reading.
Most states require uncorrected visual acuity of 20/40 or better. Of all nearsighted people, about 90% have corrections of less than –6.00 diopters (or 20/60).
Degrees of Myopia
Mild Myopia <-3.00 diopters
Moderate Myopia -3.00 to -6.00 diopters
Severe Myopia -6.00 to -9.00 diopters
Extreme Myopia >-9.00 diopters
A common tag-along with myopia, astigmatism is a condition where the cornea is irregularly shaped — more like a football than a basketball. This causes light rays to be unequally bent, coming into focus at different focal points. Objects are often distorted or "tilted," both in close and distance view.
Astigmatism is also measured in diopters. At least half of all people who are nearsighted also have astigmatism, and most of those people only have a mild degree of astigmatism at that.
Degrees of Astigmatism
Mild Astigmatism <1.00 diopters
Moderate Astigmatism 1.00 to 3.00 diopters
Severe Astigmatism 2.00 to 3.00 diopters
Extreme Astigmatism >3.00 diopters
Commonly called "farsightedness", hyperopia is a condition where the cornea is either too flat or the eye is too short. This causes light rays to come into focus behind the retina. Objects at a distance are often seen more clearly than the distorted images in close view.
Hyperopia, along with myopia and astigmatism is also measured in diopters. People who are farsighted, however, have a unique ability that other people with visual errors don't -- they can use their focusing muscles to "pull" the image forward onto the retina, due in part to the natural aging of the eye. When presbyopia (see below) sets in at around age 45, a person who had severe hyperopia can usually see distant objects much more clearly than close objects.
Degrees of Hyperopia
Mild Hyperopia <2.00 diopters
Moderate Hyperopia 2.00 to 4.00 diopters
Severe Hyperopia 4.00 to 6.00 diopters
Extreme Hyperopia >6.00 diopters
If you know someone who found the need for reading glasses or bifocals as they began to age, when they previously needed no glasses, then that person most likely is experiencing some degree of presbyopia.
Presbyopia occurs as the eye ages and the natural lens of the eye becomes less flexible. The lens's flexibility helps it move from focusing far objects to focusing close objects. This usually occurs around the age of 40-50, although since everybody experiences this condition at one time or another, that age may vary.
Although the excimer laser has no effect on presbyopia since the problem is with the lens, people with mild myopia may actually have an advantage once this condition develops. They may be able to remove their glasses when presbyopia sets in and be able to read clearly due to the counteractive effect of both conditions.
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