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| 28 Sep 2013 08:29 AM
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20/20 vision is a term for normal visual acuity (clearness of vision) measured at a standard distance of 20 feet (6 meters). Normal visual acuity would be 20/20 meaning the ability to see at 20 feet what should normally be seen at 20 feet. If the vision is worse than that, it would be expressed as 20/30 indicating the need to be 20 feet from an object to see what a normal eye should be able to see at 30 feet. The opposite could also occur in which case vision would be expressed as 20/15 or 20/10. This means the individual is able to see at 20 feet what a normal eye should be able to see at 15 feet or 10 feet.[1]
A Snellen chart is the most common instrument used by doctors and opticians to test visual acuity. It contains rows of lines of letters in varying sizes, each row having half the size of the row above. The smallest line is considered "20/200", where a person with normal vision can read the letters clearly, while a person with 20/200 vision must sit 20 feet from the chart to clearly read the line. If the person being tested cannot read even the largest letter on the chart, their visual acuity is recorded as "20/XX" (the "X" indicating that they were unable to read anything). A person with a vision of 20/40 will be able to read the letter clearly at a distance of 40 feet.
The Snellen chart was invented by Hermann Snellen of the Netherlands who published it in 1862. It has been the standard in clinical practice ever since. It has been adapted to other languages, and even to Braille for the visually impaired.
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