How is visual acuity measured with the Snellen chart?

A Snellen chart is placed at a standard distance of 20 feet away in the US (or 6 meters in the rest of the world). A person is then asked to read the letters on each line, starting at the top. If they can read the smallest line that a person with normal acuity can read at a distance of 20 feet, then they are considered to have 20/20 vision. If, however, they do not have normal vision and some of the smaller lines are blurry, then their visual acuity is determined by the smallest line they can read. For example, a person who has 20/40 vision, determined by using a Snellen chart or Snellent test, means that they can only read at 20 feet the same letters that a person with “normal” vision would be able to read at twice the distance, or 40 feet away. The largest letter on a Snellen eye chart represents a visual acuity of 20/200, which is sometimes referred to as being “legally blind.”