This week's topic is something I get asked almost everyday. Unfortunately, the answer is never as simple as it may seem. I will do my best to explain it here.
Simple definition: 20/20 is the size of letter that the average person should be able to see when fully corrected (assuming that there are no visual opacities or retinal problems). It is what we aim to achieve when correcting patients with contact lenses, glasses, and laser surgery.
More specifically: A 20/20 letter is exactly 8.726mm in height at a distance of 20 feet.
Q: Does 20/20 mean "perfect vision"?
A: No, this is a common misconception. There really isn't such a thing as perfect vision. The sharpest estimated human visual acuity is about 20/8. Which, with a little math, equates to a minuscule 3.49mm letter at 20 feet!
Q: How is the size of the letter determined?
A: The letter size for 20/20 is based on the average spacing of the light-sensing cells in the retina, thus the average eye's ability to discern spaces between letters
Q: Why 20 feet?
A: That is the approximate distance at which the internal focusing system of the eye (used for near vision) is relaxed thus allowing your eyes to effectively see into the distance.
Q: What if I can't see any of the letters without my glasses? Does that mean I'm Legally Blind?
A: Vision without correction does not really provide any significant information about your eyes (other than the fact that correction may be needed).
The definition of Legal Blindness in Canada and USA is 20/200 in the better eye with correction. Thats 10 times the size of a 20/20 letter.
Q: Do I need to see 20/20 to drive?
A: No. This may come as a surprise. But, the legal requirement for driving is 20/40 in one eye. That's double the size of a 20/20.