What is 20/20 Vision? (It’s not what you think)

Overview of 20/20 Vision    

Having 20/20 vision sounds like an achievement of excellence perfection, clarity but it really doesn't mean any of those things, in fact, it's one of the most easily confused and misunderstood aspects of all eye and vision care.

 

The last time you went to have your eye exam or vision check of vision - an ophthalmic technician or an ophthalmologist likely started off testing your vision by you looking at a board or some sort of a chart on the wall about 20 feet away asking you to read as far down as you possibly could. That chart is likely known as the Snellen chart.

Snellen Chart
Image Source- Wikipedia | Author-Jeff Dahl(CC BY-SA 3.0)

Snellen chart was originally developed in 1862 by an ophthalmologist named Herman Snellen now, it's certainly not the only chart that's out there it's maybe the most common but we have a lot of different vision charts out there.

 

1. There is an ETDRS chart used for scientific studies.

ETDRS Chart
Images source- Flickr | Author - National Eye Institute

2. There are charts with cool shapes and different symbols that we use for little children.

3. There is a tumbling E chart which is for people who can’t tell what letters they're seeing or maybe they just can't read at all maybe they just don't know the alphabet but instead they can just hold up their fingers and tell us which way the ‘E’ is pointing is that ‘left’is it ‘right’ is it going ‘up’ is it going ‘down’ there's a lot of creative ways to test people's vision.

 

But as people read down this chart getting to the smaller and smaller lines and you will start to see one of the smaller lines being the 20/20 letters.

 

20/20 is known as what is called a Snellen fraction which is the ability to see or identify a letter or some image of a certain height at a certain distance.

 

Did you know!

Outside the United States it's not even called 20/20 it's called 6/6 due to the metric system where 6 meters is roughly equivalent to 20 feet.

 

Ultimately the 20/ 20 or 6/6 describes the visual angle of an object or an image that enters the eye.

Visual Angle of the Eye If you are looking at a tree in the far distance and you walk toward that tree the tree starts to look larger and it's not that the tree actually grew larger in front of you right away it's that the tree actually just starts to take up a larger visual angle inside the eye and more of your visual field and therefore it appears larger.

 

Meaning of 20/20 Vision

20/20 vision is a vision when a person was able to identify a spatial pattern that subtends 5 minutes of arc at 20 feet. Basically, there are 360 degrees in a circle take one of those degrees to divide it by 60, and that gives you a minute of Arc

All the letters on the 20/20 line are actually designed with that in mind.

For example, the letter ‘E’ will have three arms and each arm will contain one minute of Arc including the spaces between those arms to give a total of five minutes of Arc.

 

But largely 20/20 vision is known as a standard or a common visual acuity that is obtained by the human eye and the entire visual system under normal development.

 

There are people who have a vision above that standard and below that standard. In these cases, acuity is denoted by 20 over another number such as:

Somebody who sees worse than 20/20 and they are seeing something like 20/40. In this case, the 20 on the top of the fraction describes a testing distance of you or the person that we are measuring versus the lower number which is describing the testing distance of a fictitious person with the 20/20 standard vision.

 

Comparison between 20/40 and 20/20

CASE 1:  Person with 20/40 Vision

20/40 Vision

 

CASE 2:  Person with 20/20 Vision

20/20 Vision

One more example if you were only able to see the 20/200 letter that's the largest letter ‘E’ at the top of most Snellen charts then that means that you at 20 feet could only see that large letter and that would be to note the 20 at the top of the fraction where a person who has normal standard 20/20 the vision would have to stand 200 feet away 10 times the distance away to just barely see that letter at the top of the chart.

If you are somebody that can only read that 20/200 letter even with corrective lenses then you would technically qualify to have legal blindnessaccording to the United States

 

 

Conclusion

The important conclusion of this topic is not to get hung up or anxious about seeing 20/20 every time you have your vision checked it's really just a benchmark or a standard and there are people who have worse than 20/20 vision seeing 20/30, 20/40 walking around and are very happy and with their the vision they're still able to do all the activities they want to do in their life and go where they need to go.

 

The times when it is super important to be seeing 20/20 is whenever somebody is operating a motor vehicle and that's because if you're not seeing well and operating a more vehicle you could be putting people's lives at danger.

 

It's also been found that poor school performance is directly linked to poor visual acuity and reading skills

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