When people hear the term 20/20, they generally imagine perfect vision—but that's not quite the case. Having 20/20 vision means that you can stand twenty feet from one of those iconic eye charts topped with the big E and successfully read the fourth line from the bottom without the aid of contacts or glasses. And while we aren't suggesting that this isn't a pretty significant feat—what it means is that you have amazingly normal vision.
The expression 20/20 is a Snellen fraction, named for Dr. Herman Snellen who developed the visual acuity (sharpness and clarity) measuring system in 1862. The top number never changes; it's always 20, which is the distance in feet (in the U.S) that the person is from the eye chart. The bottom number indicates how clearly you see things at a distance. If you're unable to read the fourth row from the bottom and you move up a row, you have 20/25 vision. Another row up, you're at 20/30 and so on. The higher the number on the bottom, the weaker your vision. On the other hand, if the bottom numbers go down, your vision is superior to the proverbial 20/20.
Since acuity tests only measure one aspect of vision, people with 20/20 vision may still require glasses. According to Dr. Aizuss, "Patients who are middle-aged—early 40s and older—may still have excellent uncorrected distance vision but require glasses for reading. Everyone loses their ability to focus at near as they age. This is called a loss of accommodation or otherwise known as presbyopia." Unhealthy eyes due to diabetes, high blood pressure, or glaucoma can also cause someone with 20/20 vision to need glasses. So, regardless of your number, make sure you're keeping eyes healthy by visiting your eye care professional annually.