You have most likely heard the expressions visual acuity and 20/20 vision. Yet, do people understand what these terms actually mean? When you really understand these terms, you will get why your eye doctor asks you to do more than simply read an eye chart.
The term 20/20 vision actually refers to the sharpness of sight from 20 feet away. When you have 20/20 vision, that basically means that from a distance of 20 feet you can clearly see what normal-sighted people can see 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 stand as close as 20 feet away.
Your eyes are tested separately. During the part when you're asked to read the letters on the eye chart, the smallest letters you can properly see determine the visual acuity of the eye being examined.
But 20/20 sight actually doesn't mean you have perfect eyesight, because it only assesses your clarity of vision at a distance. There are several other important vision skills; the ability to focus on close objects, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision - these are all really important to your overall eyesight. Also, someone with 20/20 vision can have unhealthy eyes. Those with damage to the sensory nerves inside their eyes as a result of diabetes, high blood pressure, glaucoma, or numerous other conditions are still able to have 20/20 vision without glasses. This is why your optometrist always performs a comprehensive eye exam, and not just a regular eye chart exam.
The next time you book yourself in for a comprehensive eye exam, you'll know what we're looking for when we ask you to read letters from an eye chart!