Visual acuity is a measure of how well a person can focus on a distant object. To test visual acuity, the person may read letters or symbols on a wall chart (Snellen test) from the top down, line by line, covering first one eye and then the other.

A visual acuity score is a ratio comparing the person's performance to that of people who have "normal" vision. A person who can read what people with normal vision can read at 20 ft (6 m) has 20/20 (6/6) vision. The larger the second number, the worse the vision.

• If vision is 20/40 (6/12), the person can read at 20 ft (6 m) what people with normal vision can read at 40 ft (12 m).
• If vision is 20/60 (6/18), the person can read at 20 ft (6 m) what people with normal vision can read at 60 ft (18 m).