It can be confusing to understand the standard way of reporting vision. Visual acuity is the measure of how someone sees.
The most common way of measuring vision is by using a Snellen chart, the common chart with the big “E” at the top. The Snellen chart is more than just random letters placed on a background. Each letter is precise in size and in the small details of the letter. For instance, the three horizontal lines that make up the E are the same size as the space between those lines. And, each letter has a height that is very specific, down to the millimeter.
In 20/20 vision, often considered perfect vision, the top number 20 refers to the distance at which the test is taken. As you probably already know, most eye doctor offices aren’t 20 feet long. Mirrors are used to cut the viewing distance in half, simulating 20 feet.
Nowadays, we use very technical LCD screens with optical software that can be placed at a distance that fits the room length since the software shrinks the letter to appear as if it’s viewed at 20 feet. The bottom number 20 refers to the smallest angle of resolution the eye can see and is comparative to other levels of vision. For instance, a person who’s vision is 20/20 sees letters twice as small at 20 feet than a person with 20/40 vision. Or put another way, a person with 20/20 vision can be twice as far away as a person with 20/40 vision to see the same target.
There are certain nuances and criteria that differ whether a person is near- or far-sighted, but the explanation here is a good representation of the measurement of vision and visual acuity.