What does it mean to have 20/20 vision?
When you have an eye exam, the doctor will state your vision is 20/20 or 20/something. What does that mean? Is 20/20 “perfect vision”? If not, what is “perfect vision”?
Let’s take a closer look at how your vision is assessed when you have an eye exam and what the terminology the doctor uses actually means.
When you visit your eye doctor and they have you read the letters on the eye chart, they measuring your visual acuity. This is a measurement of the sharpness of your vision on a standardized chart from a specified distance. This is a static test...you are seated still and the chart is stationary. Most commonly, you are viewing a bold, black letter on a white background. This is a high contrast design.
When measuring your visual acuity, the goal is to quantify the ability of your eye’s focusing system to produce a sharp, crisp image on the center of the retina, the macula.
There are certain factors that influence the level of your visual acuity.
- How well the tear film, cornea and lens of the eye focus the light
- The ability of the nerve fibers in the back of the eye to absorb the light and then send the information to the brain
- The ability of the brain to process the information that it receives
Your vision is quantified in a Snellen fraction like 20/20 or 20/40 for example. The size of the letters on each line of the eye chart is standardized. The “20/20 line” is considered “normal vision”. Measurements such as 20/30 or 20/80 are considered below normal. Measurements such as 20/15 or 20/10 are considered better than normal vision.
To think of this in another way, someone with 20/20 vision is able to see the same size letters on the eye chart at 20 feet as someone with “normal vision”. A person who can see 20/40 is able to see the same size letter at 20 feet that someone with “normal vision” is able to see at 40 feet. Someone who can see 20/15 is able to see at 20 feet what someone with “normal vision” can see at 15 feet.
You may ask...“What is the best vision a person can possibly have?” The human eye is not capable of seeing better than around 20/10 if everything is perfect. This is due to how the light receptors in the retina are arranged. The light receptors of birds of prey, like eagles, are arranged in a manner that allows them to achieve levels of vision superior to humans.