You have most likely run into the terms twenty-twenty vision and visual acuity. As common as these terms may be, do most people actually grasp their meaning? Understanding them will help you appreciate how an optometrist assesses your vision during your eye exam.
The term 20/20 indicates the accuracy of vision measured at a distance of 20 feet. If you've been told you have 20/20 vision, that basically means that from a distance of 20 feet you're able to clearly see what normal-sighted people can see from that distance. Are you aware that 20/20 is just a standard measurement? Actually, a considerable number of people have vision that's better than 20/20; for example, vision that measures 20/15, so what they would be able to see at 20 feet, a person with normal vision would only be able to discriminate from 15 feet.
Your eyes are examined separately. When the optometrist instructs you to read the letters on the eye chart, the smallest letters you can clearly read determine the visual acuity in the eye being evaluated.
But 20/20 sight doesn't always mean your vision is perfect, because it can only judge how sharply you see at a distance. There are lots of equally vital sight skills; the ability to focus on objects in your immediate surroundings, contrast sensitivity, peripheral vision, eye coordination, depth perception and color vision – these all contribute to your overall ability to see. And actually, a patient with 20/20 vision may still have unhealthy eyes. Even people who have damage to the sensory nerves inside their eyes from diabetes, high blood pressure, glaucoma, or a range of other diseases might still have 20/20 vision without needing to wear eye glasses. For this reason, an optometrist will always carry out a comprehensive eye exam, and not just a regular eye chart examination.
The next time you book yourself in for a comprehensive eye exam, you'll understand why we're asking you to read letters from the eye chart, and more!