# What is 20/20?

/While 20/20 is a term that most people widely use to describe “perfect” vision, very few truly understand what those numbers refer to. It is actually more interesting than you might think. The term 20/20 is a ratio used to describe visual acuity, or the ability to discern fine detail. The top number refers to the testing distance. It is standard in the US to test at 20 feet. (Europe tends to use 6 meters) the lower number refers to the letter size and is actually somewhat arbitrary as you will see here: A “20 foot letter” refers to a letter that subtends 5 minutes of arc. For those of us who don’t do math problems in our spare time, let me elaborate. If you look at a letter “E” from the 20/20 line, and traced an imaginary line from the top and bottom of the “E” to the center of your eye forming a long narrow triangle with the apex at the center of the eye, the angle formed at your eye would be 5/60th of a degree, or 5 min of arc. Why they chose 5 minutes of arc doesn’t entirely make sense. Most “perfect” eyes can actually discern 4 min of arc, closer to 20/15. The nice thing about the visual acuity ratio, however, is that you can theoretically test from any distance. If you stand 30 feet away from the chart and use a 30 foot letter size, 30/30 vision the ratio of 1:1 still yields the same acuity as 20/20 since at 30 feet the bigger 30 foot letter would still subtend the same 5 min of arc: stuff that is farther away needs to be bigger to see. If we keep the testing distance the same, as we traditionally do in optometry, the relative size of the denominator on the bottom of the fraction denotes how “good” the vision is. 20/40 vision means that the smallest letter you can see is twice as big as a 20/20 letter. 20/200 vision means the smallest letters you can read are 10x larger than the 20/20 letters. Conversely, you could say that if you have 20/40 vision, you would have to stand at 10 feet to see the same letters a person with 20/20 vision standing at 20 feet. So after reading that explanation, I don’t blame anybody for simply saying that 20/20 is very good vision. Don't fret the details.