It's safe to assume that you've stumbled upon the expressions visual acuity and 20/20 vision. But what do these terms really mean? Understanding what they imply will enlighten you as to how your optometrist assesses your vision in an eye exam.
20/20 actually refers to the sharpness of your vision from 20 feet away. If you have 20/20 vision, it means that from a distance of 20 feet you're able to properly see that which normal-sighted people can see from that distance. You may not know this, but 20/20 isn't the best possible visual acuity. A large number of people can even see better than 20/20; for example, vision that measures 20/15, so what they could see at 20 feet, a person with normal vision might only be able to discriminate as close as 15 feet.
Each one of your eyes is tested separately. When your optometrist instructs you to look at the eye chart and read out the letters, the smallest row that you can read accurately indicates the visual acuity in the eye that's being examined.
But 20/20 sight doesn't always mean you have perfect vision, because it only indicates how well you see at a distance. There are several other crucial sight skills; your ability to focus on objects that are close by, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these all contribute to your overall ability to see. Furthermore, a person who has 20/20 vision can certainly have unhealthy eyes. Even people who have damage to the sensory nerves inside their eyes due to glaucoma, diabetes, high blood pressure, or a range of other diseases might still have 20/20 vision without needing to wear eye glasses. For this reason, an eye care professional always conducts a comprehensive eye exam, and not just a simple visual acuity examination.
When you're having your next eye exam, you'll know exactly why we're asking you to read letters off an eye chart, and more!