The equivalent 20-foot working distance for the example above is:

• A. 20/400
• B. 10/800
• C. 20/1600
• D. 5/2000

Answer: (C)

When you want to express what you’re seeing at a nonstandard working distance in terms of the standard 20-foot Snellen scale, you convert using the magnification created by the shorter distance. The eye sees larger details when the working distance is shorter, so you scale the 20‑ft reference by how many times larger the image appears.

The magnification relative to 20 feet is roughly 20 ft divided by the actual working distance in feet. Multiply that magnification by 20 ft to get the equivalent distance on the 20‑ft Snellen scale.

If the example uses a very short working distance (giving about 80x magnification), then the equivalent 20-foot distance is 20 × 80 = 1600 ft, which corresponds to 20/1600. Therefore the equivalent 20‑foot working distance is 20/1600.