Adapted from the National Eye Institute, U.S. National Institutes of Health. Available online: http://www.nei.nih.gov

A Snellen chart is used to test far vision. It has several rows of letters. The letters on the top line are the largest; those on the bottom line are the smallest. You stand 20 ft (6 m) from the chart, cover one eye, and read the smallest row of letters you can see on the chart.