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.