The letter chart that starts with a large "E" is named after Dutch ophthalmologist Herman Snellen, who developed it in 1862. It measures visual acuity — the sharpness of your central vision at a distance.
What 20/20 means
In the US, "20/20" means you can see at 20 feet what a person with standard acuity sees at 20 feet. "20/40" means you need to be at 20 feet to see what a standard viewer can see from 40 feet. Metric countries use 6/6.
Interesting facts
- The letters used are called optotypes and are designed to be equally legible.
- Acuity is only one part of vision; it does not measure depth perception, colour vision or peripheral vision.
Read more about visual acuity.
