An eye chart test assesses general vision using a chart with letters arranged in rows. The smallest line a person can read indicates their visual acuity, with 20/20 being standard. The test is conducted at a set distance from the chart, usually 20 feet in the US and 6 meters in the UK.
An eye chart test is a type of vision test usually conducted by an optometrist and uses an eye chart or Snellen chart. This type of test is often used for situations where general vision is tested to determine how well a person can see. Other forms of testing are often used to indicate precise visual acuity, usually to determine a prescription for corrective lenses. An eye chart test consists of a person standing a set distance from the eye chart and reading the letters on the chart. The smallest size you can read indicates how well you can see.
Often referred to as an eye test, an eye chart test is primarily about using an eye chart to assess how well a person can see. An eye chart, also called a Snellen chart after Hermann Snellen who created it, usually consists of a large white sheet or poster with a series of black letters on it. These letters are arranged in rows from the top to the bottom of the graph. The top row consists of a single very large letter, while the bottom row consists of many much smaller letters.
Someone doing an eye chart test starts the test at a set distance from the chart itself, which is usually posted on a wall. In the US, this distance is 20 feet (about 6.1 m), while in the UK it is 6 meters (about 19.7 feet). When the required distance is not available a graph with each letter reversed can be used and someone can look at the graph using a mirror to create the required distance in the reflected space. The person taking the eye chart test then begins reading each letter, usually starting at the top of the chart with the largest letter and moving down as they read each line.
An eye chart test indicates a person’s visual acuity based on the lowest line they can read. The lowest line that a person with standard vision can read clearly is marked 20/20, indicating that at a distance of 20 feet (about 6.1 m) they can read it. In the UK and other areas that use metric measurements, this line is marked 6/6 to indicate the same idea in metres. Those letters that are twice this average row are marked 20/40. These letters can be read during an eye test by someone with average vision up to 40 feet (about 12.2 m) away, but someone with 20/40 vision needs to be closer, at 20 feet (about 6.1 m), to read them effectively.
Similarly, someone who takes an eye chart test and scores 20/100 would have very poor eyesight requiring no more than 20 feet (about 6.1m) distance to read something that someone with average eyesight could read 100 feet (almost 30.5 m ) away. A score of 20/200, or 6/60 in the UK, typically indicates that someone is legally blind. For someone to be considered legally blind, however, they would need to score this low while wearing corrective lenses.
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