It's likely that you have heard the terms visual acuity and 20/20 vision. But what do these terms truly mean?
20/20 indicates the accuracy of vision from 20 feet away. When you have 20/20 vision, it means that from 20 feet away you're able to accurately see that which normal-sighted people can see from that distance. So, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have be 20 feet away.
Each eye is tested on its own. When the optometrist instructs you to read the letters on the eye chart aloud, the smallest letters you can clearly see determine the visual acuity in the eye being tested.
It's important to recognize that 20/20 sight doesn't always mean that your eyesight is perfect, because it can only judge how good your eyesight is at a distance. Other crucial components to seeing properly; the ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision - these also contribute to your overall vision. Also, someone who has 20/20 vision can certainly still have unhealthy eyes. Even those who have suffered damage to the retina as a result of diabetes, high blood pressure, glaucoma, or a range of other diseases might still have 20/20 vision without glasses. For this reason, your optometrist will always carry out a comprehensive eye exam, rather than just a plain eye chart examination.
When you're having your next eye exam, you'll understand why we're asking you to read letters off an eye chart, and more!