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Visual acuity is measured in terms of the smallest line a patient can read from an eye chart when standing twenty feet away from the chart. Eye exams typically include a testing to determine the patient's visual acuity without corrective eyewear, such as eyeglasses or contact lenses. A patient who is 20 feet away from the eye chart and able to read the 20 line on the chart has 20/20 vision. Patients who can only read the larger 40 line have 20/40 vision which is worse visual acuity, while someone who can read the smaller 10 line has 20/10 vision indicating better than average visual acuity.
Consider a patient with 20/200 vision. This means they were only able to read the 200 line on the eye chart. Additionally, it means a patient with normal vision would be able to read this same line from 200 feet away.