20/200, 20/20, 20/40, 20/15, 20/10 Vision
What is 20/20 vision and it’s counterparts including 20/200, 20/40, 20/15 and 20/10?
First I’ll explain what 20/20 vision means:
The top 20 is talking about the testing distance of the eye chart. Twenty feet is the distance inbetween an eye and what it is looking at that they eye chart measures.
20 feet was chosen as the testing difference because it is the shortest distance test that human eyes do not need to focus to see.
Technically it is the shortest distance that fits “optical infinity” , meaning your eyes should not need to focus to see 20 feet away if they are working properly. Because 20 feet fits human optical infinity, it is the distance chosen to test on an eye chart.
The bottom letter refers to the size of the image you are looking at. The standard image testing size was assigned a size of “20” which is why it is called 20/20 vision.
20/20 is normal vision and stands for = 1 , as you may know if you have taken any basic math classes. This allows eye doctors to have a standard unit of 1 that represents normal vision, and deviation s of that represent vision that needs correction.
For example, 20/40 vision = .5, meaning your vision is half what a normal person’s vision is, or half what the standard vision unit of measurement is.
20/40 vision means you need to see an image that is twice as big as the standard image in order to see it while you are standing the standard distance of 20 feet away.
Another example would be that a person with standard vision can see an image 40 feet away that someone with 20/40 vision would need to be standing 20 feet away to see. Their vision would be effectively half as good in all distance circumstances if it was measured correctly.
20/200 vision means you are almost legally blind and can barely see anything, as a person with normal vision would be able to see something at 200 feet away that someone with 20/200 vision would have to be only 20 feet away to see.
On the flip side, 20/15 vision is even better than the vision of a normal person, as someone with this vision would be able to see an image clearly standing 20 feet away that a normal person would have to move up to 15 feet away to be able to see clearly.
It is the same with 20/10 vision, only this is even better vision. Someone with 20/10 vision could be standing 20 feet away and see an image perfectly that someone with average 20/20 vision would have to move up to 10 feet away in order to see the same image clearly.
Interestingly, eye charts were specifically made with letters that are easier to distinguish and see at a distance or close up, depending on the eye test. For example, the letter L is easy to see far away and a person with good vision will not easily confuse it with another letter, so it is usually used as one of the starting letters on the eye chart.
Letters that are easily confused with others such as E, are still used in the eye charts, however only one hard letter and one super easy letter is usually used per line. And the eye specialist will take it into account if someone only gets the super hard letters wrong, as opposed to when they get the super easy letters wrong when finalizing their vision prescription.
If someone only gets the hard letters wrong, the eye doctor will interpret their vision as being better than someone who gets the middle or easier numbers wrong.