It's likely that you have heard the expressions 20/20 vision and visual acuity. Still though, do people understand what these terms really mean? Having a proper understanding of them will give you insight into how an eye care professional assesses your vision when you have an eye exam.
20/20 is used to indicate the clarity of sight measured at a distance of 20 feet. When you have 20/20 vision, it means that from a distance of 20 feet you can clearly see that which is normally seen from that distance. So, 20/100 eyesight would indicate that you'd need to be as close as 20 feet away to see what a person with normal vision would see from 100 feet away. Obviously, in this scenario, you would be very near sighted.
Both eyes are examined one after another. When the optometrist instructs you to read the letters on the eye chart, the smallest row that you can read accurately determines the visual acuity of the eye that's being examined.
It's important to recognize that 20/20 eyesight doesn't necessarily mean your vision is perfect, because, after all, it only determines your distance vision. There are several other necessary sight skills; being able to focus on objects that are close by, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these are aspects of good vision. Furthermore, someone with 20/20 vision may still have unhealthy eyes. Even those who have suffered damage to the nerves within their eyes from diabetes, high blood pressure, glaucoma, or other diseases can still have 20/20 vision without needing to wear eye glasses. For this reason, an optometrist should always conduct a comprehensive eye exam, as opposed to just a simple visual acuity exam.
So the next time you have a comprehensive eye exam, you'll understand why you're being told to read letters off an eye chart, and more!