What does 20/20 mean for the eyes?

Question

I don't understand what it means to have a doctor tell you you have a 20/40 eyesight(this # may not be correct eyesight vision because I don't know what numbers it can be but its an example) What does it stand for. What is the 20 and why is it being compared to a 40

I'm using 20 and 40 as an example

Answer 1

the top number means the distance of sight, the bottom what it seems to you.

So 20/40 means something 20 feet away from you SEEMS to be 40 feet away. 20/20 means you see things normally.

That is the most basic explanation.

Answer 2

20/20 is normal vision. It is literally saying that you see at 20 feet is what someone with perfect vision sees at 20 feet. Lets say you had 20/40 vision: it is saying that what you see at 20 feet is what someone with perfect vision would see at 40 feet. If you had 20/10 vision, it means that what someone with perfect vision sees at 20 feet, you see at 10 feet. When the second number is higher, it means that you have less than perfect vision. When the first number is higher, you have better than perfect vision. The first number represents what perfect vision sees, or the distance that you are at. The second number is what you see.

Hope I helped

Answer 3

It means that you can see letters 5' of arc at a distance.

A person with 20/40 can see lettters of 10' of arc.

Answer 4

The top number is the distance you are away from the chart and the bottom number is the smallest size of letters you can read.

20/20 is 20 feet from the chart and reading the "20" size letters.  20/40 is 20 feet from the chart and reading the "40" size letters.  If the top number is smaller than the bottom number you vision is worse than average, if the top number is bigger than the bottom number is vision is better than average.

The letter size is determined by the size of letter that subtends 5" of arc at the that distance.  So the "20" size letter is 5" at 20 feet, the "40" size letter is 5" at 40 feet etc.

Answer 5

Here is a simple example.  You have 20/40 vision, and your friend has 20/20 (normal).  You can read a street sign from 20 feet away, while your friend can read the same street sign from 40 feet away.  The first number is what you can see at 20 feet.  The second number is how far away someone with normal vision can see the same thing.

~Li-Gi~ is right that 20/10 is better than normal vision, but she has the explanation backwards.  You are always the first number, 20.  If you have 20/10 vision, what you see from 20 feet away, someone with normal vision would have to be 10 feet away to see.