What is 20/20 vision? What is 20/40 vision?

20 / 20 Vision

Eye doctors frequently use the term 20/20 to describe our vision, but what exactly do they mean and how is it measured? The numbers do not actually relate to each eye respectively, as you might logically expect. 20/20 refers to how well a person can see at a distance of 20 feet. It is a term used to define normal visual acuity; the clarity and sharpness of an object at a distance of 20 feet. Often eye tests are performed in small consulting rooms that are certainly not 20 feet in length. Fortunately, with modern technology, the doctor can use a series of mirrors to project the letters onto the screen to give the same effect as a distance of 20 feet. If the patient sees less than 20/20 vision, the doctor will work out the cause. Once the problem has been diagnosed, glasses or contact lenses may be prescribed so that the desired 20/20 vision can be attained.

What the Numbers Mean
In a standard eye test the letters get progressively smaller to assess the patient's vision. The smaller the letter, the smaller the second number will be. The smaller the second number, the better vision you have, so 20/20 is not actually perfect vision; it is just what a normal person can see at that standard distance. For example, if you have 20/100 vision it means that at 20 feet you can only see what a normal person can see at 100 feet, so your eyesight would be very poor. 20/10 vision would be an excellent result as you would have twice the normal acuity, being able to read at 20 feet what a normal person would be expected to read at 10 feet. This is usually the maximum acuity the human eye can reach without using aids such as binoculars. Interestingly a hawk, which is known for its sharp vision is believed to have an acuity of 20/2 which is far better than unaided human eyesight. On the other side of the scale a newborn baby has an estimated visual acuity of 20/400, typically (not in all cases) developing to 20/20 by the age of 2 years old. Obviously other tests have to be used to assess those who are too young or handicapped to read a letter chart.

What is 20/20 vision?  What is 20/40 vision?

Definition of Blindness
If a person cannot achieve 20/200 vision or better in their best eye, even with glasses, they are considered legally blind in the USA. Also someone whose visual field is narrower than 20 degrees meets the definition of being legally blind.

Going Metric
The test and the term are measured in feet, but in Europe and many other countries distance is only measured in meters. The metric equivalent is 6/6 vision as 6 meters is the approximate equivalent to 20 feet.

The Snellen Chart
The standard eye chart used for visual acuity testing is known as a Snellen chart and was developed in 1862 by Hermann Snellen, although it has been modified several times since. The big E at the top measures 20/200 vision so reading that is no big deal. The chart has rows of letters which get progressively smaller. 20/20 vision is actually line 8 so you need to be able to read quite a few of the upper lines to have even reasonable eyesight. Eye exams test one eye at a time as our eyesight is different in each eye.

Other Factors in Determining Eyesight
As mentioned previously, 20/20 vision is a simple measurement and attaining that measurement does not mean you have perfect eyesight. There are many other factors to take into account. Vision skills also include side vision, or peripheral awareness. Someone with tunnel vision may be able to read those eye charts at 20 feet but with no side vision there is clearly something wrong. Other tests may assess the factors which can be used to diagnose more serious problems including eye disease. Eye coordination, the perception of depth and focusing ability are all part of good vision. Being able to discern color also can affect our eyesight so 20/20 is good, but it is only part of measuring good eyesight.

Farsightedness and Nearsightedness
Some people may be able to see perfectly at a distance of 20 feet; however they cannot focus on the small print on a label and may have trouble reading a newspaper. They are deemed to be farsighted. They can see distant objects clearly, but not close up. A 20/20 eye test does not allow for this. Conversely, some people cannot see objects clearly at 20 feet, but they do not realize they have a sight problem. They can read even the smallest print close up, and think they can see sufficiently well to drive. They are classified as nearsighted. An eye test may show that they do need glasses for distance. It takes a qualified optometrist and the use of several examinations and tests to perform a full eye test. Now we know that being told we have 20/20 vision is good news, but it is only part of the overall picture.

Related Questions




I've had horrible vision since I was a kid! I wonder if I ever had 20/20 vision?! Interesting article on how the 20/20 concept works.

I doubt everyone is born with

I doubt everyone is born with 20/20 vision. Some are born blind, others with vision problems from the start. The article states babies start at 20/400 vision. However it would be an interesting study to see how many people are born with perfect vision.

Acuity better than 20/20

I think the article mentions 20/20 as being the standard, believe me when I say many many people can see so much better (or further depending on your interpretation) than this. I was told as a child that I had 20/8 vision in both eyes, I am now 31 years old and never visited an opticians since then as I didn't see any need to. Although I can see detail at enormous distances, I can just as easily focus on something that is a couple of inches in front of me.

What I find interesting is that as a child I would sit incredibly close to the TV (an old CRT type back when I was growing up), I regularly read in the dark holding the books within an 2/3 inches of my eyes when my parents would turn out the lights and I went through a short phase of staring directly at the sun for as long as i could which I guess was at most 30 seconds or so.

My point being is that despite all of the above I still have 20/8 vision in both eyes, perhaps even better as it is quite easy for me to read the 20/8 line... I am not guessing any letters, they are perfectly clear.

I am no ophthalmologist but I can't think that all of this is just down to genetics. As i've exposed myseld to all of the worst environmental factors and yet I still have this razor sharp distance vision.

I have spoken to optamologists about this and I have always found they're answer/speculations rather vague. An important point is that I do not think I am alone in having this gift, I have locked eyes with people over huge distances on many occassions and it was obvious that we could both see each other with the same clarity.

I think this kind of thing goes under reported in people - like myself -
with this level of visual acuity do not go to opticians to get they're eyes tested - providing they can see up close too, aren't colour blind, have no peripheral vision problems ect... There's just no need... and it's kind of frustrating to be told that you are 20/20 or 6/6 in the UK when you know that you could easily read that bottom row from 3x the distance the chair is sat at.

I have read that 20/10 or perhaps 20/8 is the absolute limit for the human eye... I call shenanigans on this, my eyes are much better than 20/10 as I suspect many others are too, the problem is that it is rare so isn't measured for.

savant vision

Been trying to find a snellen chart that goes to 20/5 and had no luck so far. On the one i've got now, i have to put the 20/10 line 40 feet away and i can still read it clearly. So im pretty sure that puts my visual acuity at 20/5. Right now im looking at the Christmas lights on my tree and i can see the wire filament inside each individual bulb at 20 feet. Just an example of how sharp 20/5 vision is. I don't think this vision is obtainable with glasses but only with a higher density of cone cells than the normal human being.

Visual Acuity

Thanks for your reply, it's great to know that somebody else see's the world the way I do :) It's a gift for sure, i guess we both won the genetic lottery when it came to eyesight ha.

I just stumbled across this post I made a while back, and I am wondering where in the world are you from dude ? What race would you say you are from ? and interestingly.. what colour are your eyes ?

I am caucasian,from the UK, with blue eyes..i have read before that brown eyes are correlated with a lower refractive error..

In repsonse to what you said about having denser cones than the average man, i definately agree.. however I think the most important part is that the optics of the eye are essentially flawless. ie: Nature made no mistakes when making our eyes.

peace :)

Amboliopia and Strabismus

I have Amboliopia and Strabismus in my left eye and see 20/100 and my right eye is 20/20 with glasses prescription on. My recent eye exam the optometrist told me that there is nothing he could do for my left eye.

Great Article! It lucidly

Great Article! It lucidly explains what Ophthalmologists mean when they say 20/20 and breaks down the jargon for everyone to understand.

Another point of contention

WHen I was young (like 4 maybe 5 years old), the UK still used the imperial system. So eye charts were 20/20 20/30 20/40 ect... however they also went all the way down to 20/8. Nowadays we've moved to metric and they seem to have dropped the 3 bottom rows so that the bottom smallest row is 6/6 metres ie 20/20 feet.

I have read off of numerous eye doctors that most young people who can read the 6/6 line would probably have an acuity of around 6/4 - this isn't uncommon at all.

The problem is that their are people who could read the 6/6 line from 18 metres (20/20 line from 60 feet) with no bother and people who would sit at 20 feet and squint and end up guessing a few of the letters. Both of these patients would be told that they have 20/20 vision although 1 clearly has 3 times the acuity of the other.

Sorry to go on about this but as somebody who possesses this gift, it is extremely annoying that the medical profession rarely acknowledges it's existence. To people close to me... my family, close friends, work colleagues it is obvious for instance i can read a size 12 font on a standard 17" monitor from 3 rows of desks back or see the entire flight of a golf ball driven over 300 yards and watch it drop - or - at it's most useful, read a roadsign that is almost a quarter of a mile away, long before my passengers can even make out the it has text written on it.

I work in IT staring close-up at a monitor all day long, I sometimes feel this that with this gift alone that I should have trained as a pilot or maybe even been a cricket/golf/baseball player as I would have a substantial advantage over other players.

I really hope that laser eye surgery can get to the point where everyone can acheive at least 20/10 vision - if not better like 20/8 - or perhaps beyond as I think people who see in 20/20 now's jaws would literally drop at how much clearer the world looks and how you see that same clarity at enormous distances.

A closing point, no matter how far away something is, I never see it blurry, instead it's like my retina runs out of pixels and I see it as a point but it still appears sharp, there is never any blur regardless of distance.

Can anybody here relate to anything I have said ? Please tell me that I am not alone !

Peace, Dan.

Gaming improves vision

I heard action gaming actually improves vision along with brain function.

Am I legally blind or.....

My eye prescription is -4.75 that puts me between 20/200 & 20/600, right? I have glasses that correct them but I want to know should I apply for disability because sometimes I lose or break my glasses and I can not function without them. Seriously ran into my sons bedroom door because I couldn't see that it was close.

According to the article to

According to the article to which you replied, you are only considered blind if you cannot reach 20/20 WITH your glasses. Since you can reach it with your glasses, you would not qualify.

Have you thought about getting contact lenses or having laser surgery?

Is a child legally blind if

Is a child legally blind if they have 20/40 vision with glasses in one eye and can only see shadows with other eye with glasses on

In the USA, legal blindness

In the USA, legal blindness is counted based on your better eye. 20/40 vision is better than 20/200 vision, so the child would NOT be considered legally blind in this case.