What does it meant by 20/20 and 20/40 vision? Is 20/20 stated that left eye and right eye are stable?

Question

Eg: Like a person has 300 degrees on his left eye and right eye. Does it means by 20/20?

(Aviation-related question)

Answer 1

20/20 is considered "normal" vision.  20/40 means that you can see at 20 feet what someone with normal can see clearly at 40 feet, which means you are nearsighted.  20/10 means you can see at 20 feet what someone with normal vision can see at 10 feet.  This is called visual acuity.  And yes, each eye may be slightly different.

The 300 degrees thing is called peripheral vision.  This describes the ability to see out of the corners of your eyes.  Hold up your finger directly in front of you and slowly move it to the side while continuing to look straight ahead.  Your field of vision ends where you can no longer see your finger.  See the link below for normal human ranges.

Answer 2

No, I think that "20/40" for instance means that you can see something clearly from 20 feet away, whereas a person with normal eyesight can see it clearly from 40 feet away.  Or something like that.

It is possible to have, for instance, 20/20 in the left eye and 20/30 in the right eye.  So it's not that the top number is for the left eye and bottom number for the right eye.