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Visual acuity is defined as the clarity or sharpness of vision, which is the ability of the eye to see and distinguish fine details. This is an important factor for a variety of everyday tasks, including reading text, recognizing symbols, and performing assembly work. Good visual acuity is very important when driving, because it helps people recognize landmarks, avoid obstacles, and read road signs. An ophthalmologist or optometrist measures how clearly a person sees during a routine eye exam using a wall chart with symbols or letters. It is determined by the smallest line the patient can read on the chart.
This measurement is typically given as a fraction, such as “20-20” or “20-40.” The first number refers to the distance at which the patient’s vision was tested, which is generally 20 feet (6 meters). The second number indicates the distance from which a normal eye can see the symbol or letter on the chart. If a person’s visual acuity is measured at 20/40, therefore, the person can see at 20 feet what a person with normal vision can see at a distance of 40 feet (12.1 m). The fraction 20/20 is considered normal vision. A measurement of 20/200 or worse is considered legally blind.
A person’s visual acuity can be influenced by several factors, including color, brightness, and contrast. A bright light or color can make it more difficult for the eye to perceive a certain object. On the other hand, the more contrast between an object and its background, the easier it is for the eye to pick out small details. Eyesight is also affected by conditions such as shortsightedness and long sightedness.
Visual acuity is measured for a variety of reasons. An eye doctor decides on a patient’s prescription by determining which corrective lens power will permit the best clarity. Changes in eyesight can be indications of a serious eye disease. Good visual acuity is required for employment in certain occupations, such as airline pilot or police officer. In addition, many jurisdictions require people to have a corrected eyesight of 20/40 in at least one eye before they can receive a driver’s license.