We've all come across the expressions 20/20 vision and visual acuity. Still though, what do these terms truly mean? Understanding these terms will help you appreciate how an eye care professional evaluates your eyes during an eye exam.
20/20 vision indicates the clarity and sharpness of sight measured at a distance of 20 feet. If you've been told you have 20/20 eyesight, that basically means that from twenty feet away you are able to accurately see what should be seen from that distance. To give an extreme example, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have be as close as 20 feet away. Obviously, in this scenario, you would be extremely near sighted.
Each eye is evaluated separately. During the part when you're asked to read the letters on the eye chart aloud, the smallest letters you can properly see determine the visual acuity of the eye being tested.
However 20/20 eyesight actually doesn't mean that your eyesight is totally free of flaws, because it only determines how good your vision is at a distance. There are several other vital vision skills; your ability to focus on objects that are close by, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these all contribute to your overall ability to see. More importantly, a patient who has 20/20 vision can still have unhealthy eyes. Those with damage to the sensory nerves within their eyes as a result of glaucoma, diabetes, high blood pressure, or a range of other diseases can still have 20/20 vision. And because of this, your optometrist always performs a comprehensive eye exam, rather than just a regular eye chart test.
The next time you find yourself having an eye test at the optometrist, you'll understand why you're being told to read letters off an eye chart, and more!