The eye chart that most people are familiar with is called the Snellen chart. Visual acuity is the term that we as doctors use to describe what someone can see on the eye chart. The Snellen acuity format is expressed in the form of 20/___, where the top number in the fraction indicates the testing distance (20 ft), and the bottom of the fraction indicates the size of the letter.
I’m not going to get into how the size of the letter is actually calculated, but there are standard sizes for a 20 foot letter, a 30 foot letter, a 40 foot letter, and so on. The actual size of the letter is not actually 20 feet tall. Again, I won’t bore you with the details on why we call it this. Just understand that a letter designated as a 20 foot letter is a certain size, and the larger the bottom number on the fraction, the larger the letter. So…a letter on the 20/20 line is much smaller than a letter on the 20/400 line. In other words, 20/20 vision is much better than 20/400 vision because a person with 20/20 vision can resolve a much smaller letter than a person with 20/400 vision.
Here’s another way to look at this. Imagine a person with 20/20 has “normal” vision. Then take a person with 20/400 vision. How can you compare these 2 patients? The person with “normal” vision could be 400 feet away and resolve the same letter that the person with 20/400 vision could only resolve at 20 feet!