It's likely that you have come across the expressions 20/20 vision and visual acuity. As common as these terms may be, do you actually grasp their meaning? When you really understand these terms, you will see why an eye doctor needs to assess more than just how well you read an eye chart.
The term 20/20 vision refers to the accuracy of eyesight from 20 feet away. If you've been told you have 20/20 eyesight, it means that from a distance of twenty feet you can clearly see that which is normally seen from that distance. So, 20/100 vision would indicate that to see what most people can see from 100 feet, you would have to stand only 20 feet away. Obviously, in this scenario, it would mean that you would be extremely near sighted.
Each eye is tested on its own. When you're asked to read the letters on the eye chart aloud, the smallest row that you can read clearly determines the visual acuity in the eye that's being evaluated.
It's important to recognize that 20/20 eyesight doesn't always mean your vision is perfect, because, after all, it only assesses how good your vision is at a distance. There are several other important vision skills; your ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral awareness, eye coordination, depth perception and color vision – these also contribute to your overall vision. More importantly, a person who has 20/20 vision can certainly have plenty of other eye-related health problems. People with damage to the retina from diabetes, high blood pressure, glaucoma, or a range of other diseases can still have 20/20 vision without glasses. For this reason, your eye care professional always performs a comprehensive eye exam, as opposed to just a plain visual acuity exam.
The next time you find yourself having an eye test at the optometrist, you'll understand why you need to read letters off an eye chart, and more!