During eye exams, optometrists use eyesight charts to gauge patients' visual acuity by asking them to read letters from the smallest line they can see. The general guideline concerning eyesight charts is that a person with normal 20/20 vision should be able to read up to at least eight lines of text from 20 feet away. When determining eyeglass prescriptions, optometrists adjust lens strength until their patients have at least 20/20 visual acuity according to the chart.
The American Optometric Association explains that eyesight charts usually range from 20/10, which indicates very sharp vision, to 20/200, which is considered legally blind. The number indicates the distance from which a person with normal vision should be able to see a particular line. For example, for those with 20/40 vision, the smallest line of text they can see standing 20 feet from the chart is readable by a person with normal vision standing 40 feet from the chart.
Snellen charts are the most common example of eyesight charts for testing visual acuity, and they usually consist of up to 12 lines of text, as described by AllAboutVision.com. The text begins with one very large letter at the top, and it is followed by subsequent rows of letters that get smaller on each line. The optometrist sets the chart up to 20 feet away from the point of reference.