What Is 20/20?
- WHAT IS 20/20?
- I get this question all the time, and I'm always amazed at what patients think it is and/or what they've been told that it is. 1st let me tell you what it is NOT:
1) It is not your prescription. You cannot have a prescription of 20/20 or 20/50 or 20/70 or 20/anything.
2) Its not "perfect" vision. Its only average vision for a normal, healthy human eye. Most healthy eyes should be able to see better than 20/20.
3) Its not a comparison to anyone else's vision. For example I get told all the time by patients that they heard that if they have 20/40 vision its the same as having 20/20 vision but being closer...? Some jibe about "you see at 20 feet what a normal person sees at 40 feet"...No.
20/20 is an angular measurement of letter size. Thats it. The top "20" is the test distance...in the U.S. its 20 feet (in Europe they use meters, so average vision for the human eye is 6/6, with the top "6" being 6 meters). The BOTTOM "20" is the harder one to explain:
The denominator of any Snellen acuity is "the distance at which the letter subtends 5 minutes of arc". Yeah...wait, what? LOL seriously, its confusing. Remember that I started all of this by saying that we're talking about an angular measurement of letter size. If a circle has 360 degrees in it, and each degree is further divided into 60 minutes, then the denominator of the acuity is the distance that the letter size in question produces an angle that equals 5 of those minutes of one degree. Its not an easy thing to wrap your brain around.
The reason we have to use angles like that instead of talking about a 6" letter or a 5" letter or whatever, is b/c obviously a 6" letter looks really small at 200 yards, and really big if your nose is 2" away from it. We as a profession needed a way to "standardize" the vision measurements with varying test distances. The way to do that is to talk about the angles, not the letter sizes.
"Snellen" letters are generally the standard...its an optotype that is easy to distinguish.
Now...if your "prescription" cannot be "20/20", then what is your prescription?? Again we use distances. But we couldn't have made it easy...for some reason we've decided (a log time ago! lol) as a profession to make it hard/confusing. Don't know why that is...its not my fault! We talk about prescriptions in "diopters", which are reciprocal meters. In Europe this makes perfect sense, since they use 6 meters as the standard test distance. But in the U.S we use FEET for our test distances and reciprocal METERS for our prescriptions. Very backwards if you ask me.
Anyway if a person is nearsighted and can only see well out to about 40 cm (roughly 16"), then that person has a "prescription" of -2.50 diopters. 40 cm is 0.4 meters. 1/.4 = 2.5 DIOPTERS, or 2.5 reciprocal meters.
Lets do another one: if a person can only see well out to 25 cm (roughly 10 inches), then their prescription is -4.00 diopters. 25 cm = .25 m and 1/.25 = 4 diopters.
If we eye docs were "normal" (which we're not!) we'd just write prescriptions for corrective lenses as being the distance that the patient can see w/o any help. Like a "prescription" of -25 cm or -40cm, etc. That would make more sense to me. But then again I didn't make the rules!
Those examples were for nearsighted or "minus" Rx's (in the eye profession we never say "negative" or "positive", we always say "minus" and "plus", again I don't know why!) which are much easier to understand. Farsighted or "plus" prescriptions and astigmatic prescriptions are not as easy to explain. Maybe I'll try later LOL