You have most likely come across the terms visual acuity and twenty-twenty vision. As frequently used as these terms are, do you really grasp their meaning?
20/20 refers to the accuracy of vision from 20 feet away. If you've been told you have 20/20 vision, that basically means that from a distance of 20 feet you can clearly see what is normally seen from that distance. To give an extreme example, 20/100 vision indicates that to see what most people can see from 100 feet, you would have to stand only 20 feet away.
Both eyes are examined one after the other. During the part when you're asked to look at the eye chart and read out the letters, the smallest row that you can read properly indicates the visual acuity of the eye that's being examined.
But 20/20 vision doesn't always mean that your eyesight is perfect, because it only assesses your distance vision. Other important sight skills; your ability to focus on objects that are close by, contrast sensitivity, peripheral awareness, depth perception, eye coordination and color vision – these are all really important to your general eyesight. More importantly, a person with 20/20 vision can certainly have plenty of other eye-related health problems. People with damage to the nerves within their eyes from glaucoma, diabetes, high blood pressure, or other diseases might still have 20/20 vision. For this reason, an optometrist always conducts a comprehensive eye exam, as opposed to just a simple visual acuity examination.
The next time you find yourself having an eye test at the optometrist, you'll understand what we're looking for when we ask you to read from an eye chart!