What is the properly calibrated height of the 20/200 letter on the Snellen chart for a testing distance of 13 feet?

• A. 50 mm
• B. 58 mm
• C. 62 mm
• D. 70 mm

Answer: (B)

Preserving the same visual angle is what calibrates Snellen letters when you change testing distances. The 20/20 letter height at the standard 20-foot distance is about 8.87 mm. The 20/200 letter is 10 times larger, so at 20 feet its height is about 88.7 mm. To keep the same 5-arcminute subtend at a shorter distance, scale the height by the distance ratio: 88.7 mm × (13/20) ≈ 57.7 mm, which is about 58 mm. So the properly calibrated height at 13 feet is roughly 58 mm.