Everyone has stumbled upon the expressions 20/20 vision and visual acuity. Yet, what do these terms truly mean? Understanding what they imply will help you appreciate how an eye specialist determines the state of your vision in an eye exam.
The term 20/20 vision indicates the accuracy of your eyesight measured at a distance of 20 feet. If you have 20/20 eyesight, that means that from a distance of 20 feet you're able to accurately see that which is normally seen from that distance. To give an extreme example, 20/100 eyesight indicates that to see what most people can see from 100 feet, you would have to stand only 20 feet away. Obviously, if this was the case, you would be very near sighted.
Each eye is tested on its own. When the optometrist asks you to read the letters on the eye chart, the smallest letters you can properly read determine the visual acuity in the eye being evaluated.
20/20 vision doesn't necessarily mean you have perfect eyesight, because it only assesses your distance vision. There are other really vital vision skills; being able to focus on objects that are close by, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision - these are aspects of healthy vision. More importantly, a patient with 20/20 vision can certainly still have unhealthy eyes. People with damage to the retina due to glaucoma, diabetes, high blood pressure, or other conditions are still able to have 20/20 vision, without the help of glasses. This is why an optometrist will always carry out a comprehensive eye exam, as opposed to just a simple visual acuity exam.
During your next eye exam, you'll understand why you're asked to read letters from the eye chart, and more!