1 M is equivalent to which Snellen acuity at 40 cm?

• A. 20/30
• B. 20/50
• C. 20/70
• D. 20/60

Answer: (B)

Visual acuity is about the angular size of what you can resolve. A Snellen letter is defined so that the critical detail subtends 5 arc minutes at the eye for a 20/20 letter. A metric 1 M letter is defined as subtending 5 arc minutes at a distance of 1 meter. If you view that same 1 M letter at 40 cm, the angle it subtends increases by the ratio of distances: 1 meter divided by 0.4 meter equals 2.5. So the angular size becomes 2.5 × 5 arc minutes = 12.5 arc minutes. Since 5 arc minutes correspond to 20/20, an angular size 2.5 times larger corresponds to a Snellen denominator 20 × 2.5 = 50. That means 1 M at 40 cm is equivalent to 20/50.