Everyone has heard the expressions visual acuity and twenty-twenty vision. But what do these terms actually mean? Understanding these terms will enlighten you as to how an eye specialist determines the state of your vision during an eye exam.
20/20 refers to the sharpness of your vision measured at a distance of 20 feet. When you have 20/20 eyesight, it means that from a distance of 20 feet you can see that which normal-sighted people can see from that distance. Alternatively, 20/100 eyesight would indicate that you'd need to be as near as 20 feet away to see what a person with normal vision can see from 100 feet away. Obviously, if this was the case, it would mean that you would be very near sighted.
Each eye is evaluated separately. During the part when you're asked to read the letters on the eye chart, the smallest letters you can properly read determine the visual acuity in the eye that's being tested.
But 20/20 vision doesn't necessarily mean your vision is perfect, and that's because it only determines how well you see at a distance. There are lots of equally 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 aspects of good vision. Also, a patient with 20/20 vision can have plenty of other eye-related health problems. People with damage to the retina due to diabetes, high blood pressure, glaucoma, or a range of other conditions might still have 20/20 vision without needing to wear eye glasses. And because of this, an optometrist always conducts a comprehensive eye exam, rather than just a regular eye chart test.
During your next eye exam, you'll understand why you're asked to read letters from the eye chart, and more!