What does ‘20/20’ mean, and why can some people who need to wear glasses see better than some people who don’t?

This may be the most commonly asked question during an eye exam, aside from “which is better, one or two?”. We’ve all heard about 20/20, or 20/40, but what do these numbers really mean?

The two numbers both refer to distance measurements, in feet: the top number is always 20, because 20 feet is the usual testing distance when reading the eye chart during the exam. The bottom number is the distance from which a person with “average” vision is able to see the same size letter. For example, if your visual acuity is 20/20, you are able to read the same letters from a distance of 20 feet that can be read by a person with average vision at 20 feet. However, if your acuity is 20/40, you need to be at 20 feet to see what a person with average vision can see from a distance of 40 feet.

Many people assume that 20/20 is ‘perfect’, but a better description would be “average”: 20/20 is what the majority of the population is able to read when their prescription is properly corrected. Some people are able to see 20/15, while some may only manage 20/25, depending on the quality of the optics of their eye (much like some cameras have better resolution than others) … and that’s why someone with glasses may actually see a little better than someone who doesn’t need glasses at all.