You've probably stumbled upon the terms 20/20 vision and visual acuity. Still though, what do these terms really mean? Understanding what they imply will help you appreciate how an eye care professional determines the state of your eyes when you have an eye exam.
The term 20/20 actually refers to the clarity of eyesight measured at a distance of 20 feet. When you have 20/20 vision, that means that from a distance of 20 feet you're able to clearly see that which should be seen from that distance. Alternatively, 20/100 vision would indicate that to see what most people can see from 100 feet, you would have stand as close as 20 feet away.
Each eye is evaluated on its own. During the part when you're asked to correctly read letters from the eye chart, the smallest letters you can clearly read determine the visual acuity of the eye that's being evaluated.
But 20/20 sight actually doesn't mean your vision is perfect, because, after all, it only indicates your distance vision. There are other really vital sight skills; being able to focus on close objects, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision - these are aspects of good vision. Also, someone who has 20/20 vision may have unhealthy eyes. People with damage to the sensory nerves within their eyes due to diabetes, high blood pressure, glaucoma, or numerous other diseases are still able to have 20/20 vision. For this reason, an eye care professional always conducts a comprehensive eye exam, rather than just a plain eye chart test.
When you're having your next eye exam, you'll know what we're testing for when we ask you to read aloud from an eye chart!