What is the Snellen acuity corresponding to 1 M at 40 cm?

• A. 20/20
• B. 20/25
• C. 20/50
• D. 20/60

Answer: (C)

Snellen acuity is set by the angular size of the optotype that can be resolved. A 1 M letter is defined to subtend 5 arc minutes of visual angle at 1 meter. When you bring that same letter to 40 cm, its visual angle becomes larger by the distance ratio (1 m / 0.4 m = 2.5), so the 1 M letter at 40 cm subtends 12.5 arc minutes. On a Snellen chart, the line for 20/20 is sized so that the letter subtends 5 arc minutes at 20 feet. If the letter you can read at 40 cm subtends 12.5 arc minutes, it corresponds to a letter that is 2.5 times larger than the 20/20 size, i.e., 20/50.