You might have heard at your eye exam that you have 20/20 vision (great!), but what exactly do those numbers mean?
The top number refers to the testing distance your vision is checked at: 20 feet. You might think, "But the exam room isn't that long!" Well, using optics and math, eye doctors can calculate a smaller (or larger) testing distance to fit in a smaller (or larger) exam room that is equivalent to using a 20 foot testing room. Essentially the testing chart letters are adjusted for size and placement.
The bottom number refers to what normally should be seen at a distance of 20 feet. For example, if your vision is 20/20 then you can see what should normally be seen at 20 feet. If your vision is 20/400, then you need to be 20 feet away from something that should normally be seen from a distance of 400 feet.