What is 20/20 Vision? - Page 3

Eye Chart -- Japanese

Since the standard testing distance for the Snellen chart used in the United States is 20 feet, the top number in the Snellen fraction is always "20." The bottom number varies, depending on the size of the letters on each row of the chart.

Here's how it works. Three lines above the 20/20 line on the Snellen chart, the letters are twice as large as those on the 20/20 line. A person with normal visual acuity therefore can be expected to correctly identify these larger letters from a viewing distance that is roughly twice that of the normal testing distance of 20 feet (so, 40 feet).