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The ability to discriminate fine details when looking at something is visual acuity. One of the most popular ways of measuring visual acuity is with the Snellen eye chart. This measurement is often called the 20/20 test because a person with normal vision can stand 20 feet from the chart and easily identify the row of letters fourth up from the bottom. A person whose eyes cannot be corrected with glasses to 20/200 is usually considered legally blind.
In the visual impairment of myopia (nearsightedness) the eyeball is unusually long, so the image is focused in front of the retina. Glasses can be used to see normal to long-range stimuli in the environment. In hyperopia (farsightedness) the eyeball is unusually short and the image is focused beyond the retina. Glasses permit close-up work, such as reading. |
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