What Snellen acuity is represented by 1 M at 1 meter?

• A. 20/20
• B. 20/40
• C. 20/100
• D. 20/200

Answer: (A)

Understanding Snellen acuity in terms of angular size helps here. A letter sized so that its overall height subtends five minutes of arc at the eye defines the standard acuity mark. A one-meter-tall letter viewed from one meter away creates that same five-arcminute visual angle as a standard 20/20 letter does when viewed from about 20 feet (roughly 6 meters). So reading a 1 M letter at 1 meter indicates normal visual acuity, equivalent to 20/20 (6/6). If the letters had to be read only from much closer or could be read only from farther away, that would correspond to worse acuity (larger denominators like 20/40, 20/100, 20/200).