I've always had 20/15 vision in both eyes, so I've always wondered why there are so many people in the world with bad vision. Actually I can usually bring the

I've always had 20/15 vision in both eyes, so I've always wondered why there are so many people in the world with bad vision. Actually I can usually bring the word into focus using someone else's glasses- unless they are the super thick coke bottle ones, or they have a really bad astigmatism, in which case I can bring either the horizontal or the vertical into focus but not both. I wish I knew what I did right, or wrong!