You might know that you have 20/20 vision, but do you really understand what you’re saying when you use that phrase? When your optometrist tells you that set of 2 numbers, what does he or she really mean? There are a lot of misconceptions about it, but it really is quite simple.
The Snellen Chart
The stereotypical Eye Exam chart with all the letters at various sizes is called a Snellen chart. This is named after the ophthalmologist who developed the chart over 150 years ago. The concept of this chart is where the phrase 20/20 comes from. It is used to measure visual acuity, or how detailed and distanced one’s eyesight actually is. Herman Snellen (who invented the chart) defined standard vision, which is now measured using the chart.
There is a line on the chart which is the standard line. Most people with average eyesight should be able to read this line from 20 feet away with no problems, and so the chart is usually placed at 20 feet away from the patient. This is where one of the 20s comes in: it is the distance the person is away from the chart (or any object they are trying to see). Because the standard exam is given at 20 feet, this number does not change.
What Does the Other 20 Mean?
The second 20 is the distance the average person (or someone with 20/20 vision) would be able to see those letters from. Therefore, someone with 20/20 vision can see something from 20 feet away that the average person would be able to see from 20 feet away. If someone has a harder time with distance vision or small details, they might have 20/40 vision. This means that in order to see something that the average person can see from a distance of 40 feet (the second number), they must be at a distance of 20 feet.
The smaller the second number gets, the better your eyesight, because it means you can see things from further away. So although 20 is average, 40 is worse, and 15 or 10 is better. If someone has 20/10 eyesight, they can be 20 feet away from something that everyone else has to see from only 10 feet away. Someone with 20/10 vision is to someone with average 20/20 vision and that average person is to someone with 20/40 vision.