It's safe to assume that you've stumbled upon the expressions twenty-twenty vision and visual acuity. As common as these terms may be, do you really grasp their meaning? Really understanding what they imply will give you insight into how an eye specialist assesses your eyes in an eye exam.
The term 20/20 indicates the clarity and sharpness of sight from 20 feet away. If you have 20/20 eyesight, that basically means that from a distance of twenty feet you're able to accurately see that which normal-sighted people can see from that distance. So, 20/100 eyesight would indicate that to see what most people can see from 100 feet, you would have stand 20 feet away. Obviously, if this was the case, it would mean that you would be pretty near sighted.
Your eyes are tested separately. When you're asked to read the letters on the eye chart, the smallest letters you can clearly read determine the visual acuity of the eye that's being tested.
It's important to recognize that 20/20 eyesight doesn't always mean you have perfect eyesight, because it only assesses your distance vision. There are lots of equally important sight skills; the ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, depth perception, eye coordination and color vision - these all contribute to your overall vision. Furthermore, a person who has 20/20 vision can have eye problems. Those with damage to the sensory nerves inside their eyes from glaucoma, diabetes, high blood pressure, or other diseases are still able to have 20/20 vision without needing to wear eye glasses. For this reason, an eye care professional will always carry out a comprehensive eye exam, as opposed to just a simple 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 from an eye chart!