It's safe to assume that you've run into the expressions 20/20 vision and visual acuity. As frequently used as these terms may be, do you actually know what they mean? Having a proper understanding of what they imply will enlighten you as to how an optometrist determines the state of your vision when you have an eye exam.
The term 20/20 refers to the accuracy of sight from 20 feet away. If you have 20/20 vision, that means that from 20 feet away you can clearly see that which is normally seen from that distance. You may not know this, but 20/20 vision is just a standard measurement. A large number of people have vision that's better than 20/20; for example, 20/15, so what they would be able to see at 20 feet, a person with normal vision might only be able to discriminate at 15 feet.
Both eyes are examined one after the other. During the part when you're asked to read the letters on the eye chart, the smallest letters you can properly see determine the visual acuity in the eye being examined.
However 20/20 eyesight actually doesn't mean that your eyesight is flawless, because, after all, it only determines your distance vision. There are several other vital sight skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision - these are all extremely important to your overall eyesight. Furthermore, a patient with 20/20 vision can have plenty of other eye-related health problems. Those with damage to the retina due to glaucoma, diabetes, high blood pressure, or numerous other diseases might still have 20/20 vision. For this reason, your eye care professional will always carry out a comprehensive eye exam, as opposed to just a plain visual acuity test.
The next time you find yourself having an eye test at the optometrist, you'll know what we're testing for when we ask you to read from an eye chart!