An eye chart measures the sharpness of vision, which is also called visual acuity. The top number stands for the person’s distance in feet from the chart, while the bottom number stands for distance at which a person with normal eyesight is able to read the same line.
For example, if you have 20/30 vision, this means your vision is worse than average. You can read letters on this line at a distance of 20 feet, where most people can see these letters from a further distance of 30 feet.
20/20 vision does not mean the best vision possible. Some people can see better than the 20/20 line on the chart. For example, if you have 20/15 vision, this means your vision is sharper than average. At a distance of 20 feet, you can read letters that most people can see when they are closer to the chart, at 15 feet.
Glasses and contacts can help improve someone’s vision to 20/20. Without glasses, contacts, or eye surgery, approximately 35% of adults have 20/20 vision. With correction, approximately 75% of adults have 20/20 vision.