What is Visual Acuity?
Visual acuity refers to how close a person needs to be to clearly see an object that’s placed 20 feet away from them. A person with 20/20 vision can see what an average person can see on an eye chart when they are standing 20 feet away. Let’s break this down a little more, the top number in the fraction refers to the distance in feet that you stand from the chart and the bottom number indicates the distance at which a person with normal eyesight can read the same line you correctly read. So if you have a visual acuity of 20/80, it means that you’re able to see details at 20 feet away from the same object as a person with 20/20, or normal vision, could see from 80 feet away.
How Is Visual Acuity Measured?
We’ve all been asked to cover one eye and read different size numbers and letters from a chart while getting our eyes examined. That is because the eye care specialist is measuring our visual acuity. Since many doctors don’t have 20 feet of space in their office, it is common to use a projector and a mirror to display the chart instead.
The most used chart for checking vision is a Snellen Eye Chart. This is the chart with the large E at the top, that most of us have encountered at a doctor’s office. The chart was named after ophthalmologist Herman Snellen. More than 150 years after it was invented, it’s still one of the most commonly used techniques to measure a person’s visual acuity.