Everyone has heard the expressions 20/20 vision and visual acuity. As frequently used as these terms are, do most people actually grasp what they mean? When you understand them, you will see why your eye care professional asks to assess more than just how well you read from an eye chart.
The term 20/20 vision indicates the clarity and sharpness of your vision from 20 feet away. If you've been told you have 20/20 eyesight, it means that from twenty feet away you can clearly see what should be seen from that distance. To give an extreme example, 20/100 vision would indicate that you'd need to be as near as 20 feet away to see what a normal-sighted person can see from 100 feet away. Obviously, in this scenario, it would mean that you would be very near sighted.
Each eye is tested separately. When the optometrist instructs you to read the letters on the eye chart aloud, the smallest letters you can properly see determine the visual acuity in the eye being evaluated.
20/20 sight doesn't always mean you have perfect eyesight, and that's because it only assesses how well you see at a distance. There are lots of equally important components to seeing well; being able to focus on close objects, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision – these are aspects of good vision. More importantly, a patient who has 20/20 vision may have unhealthy eyes. Those with damage to the retina from glaucoma, diabetes, high blood pressure, or numerous other diseases are still able to have 20/20 vision without needing to wear eye glasses. And because of this, an eye care professional always conducts a comprehensive eye exam, and not just a plain visual acuity examination.
When you're having your next eye exam, you'll understand what we're looking for when we ask you to read letters aloud from an eye chart!