View Full Version : eye sight question

10-26-2001, 05:48 PM
What does 20/20 mean? I know it means you have perfect vision, but what are the numbers for? Can you have better than 20/20? I think you can, but I'm not sure. Any doper have better than 20/20? I have 20/20 now that I got new glasses.

10-26-2001, 05:53 PM
This has been discussed several times before so you might want to do a search. The 20/20 thing is pretty subjective and meaningless when it comes to defining what eyeglasses or correction you need which is measured in diopters.

10-26-2001, 05:53 PM
The way it was explained to me was that 20/20 means that what a person with normal eyesight can see at 20 feet, you can see at 20 feet. Ergo, if you have 20/60 vision, what a normal person can see at 60 feet, you can't see until it's only 20 feet away. The first number is always 20. It is possible to have 20/10 vision -- what a normal person can see at 10 feet, you can see at 20 feet.

However, my optometrist for the last several years has used diopters instead of the 20/20 system. Instead of telling me I have 20/60 vision, or whatever, he tells that my left eye is -4.75 diopters and my right eye is -5.25 diopters. Maybe the 20/20 system is falling into disuse? Or maybe I just have a renegade optometrist.

10-26-2001, 07:18 PM
They have *always* used diopters. It is how eyeglasses are defined. The 20/20 thing does not give any information regarding what correction your eyes need. That system is only used in the US for the public. Eyeglasses around the world are defined in terms of diopters (spheric, cylindrical etc).

2nd guest
10-27-2001, 06:45 PM
You can be corrected to 20/15. 20/20 is not the top of the scale, but just what someone considered Normal the day they made the first chart. This is akin to Farenheit setting his scale from 0 as his lowest recorded air temperature and 100 as the highest. His sample was not universal, but it was all he had to deal with.

10-27-2001, 07:40 PM
If you are only slightly myopic (near-sighted), the 20/40, 20/60 is useful. However, once your myopia reaches moderate proportions, it's meaningless to use that system. A refractive error of -4 or -5 diopters cannot easily be reduced to 20/--. I needed diopter corrections of -7 or so in both eyes before I had radial keratotomy (the knife, not the laser). Now, correction in my right eye is -2, but with astigmatism of -1. Left eye is plano, but with astigmatism of -3, which equals myopia of -1.5, as explained to me by an opthalmologist with whom I used to play tennis. He did not do the surgery, but he wanted to do the laser. I got a second opinion, and the other guy advised against it, for reasons I need not go into here. (Neither of those guys did the surgery, and both said they would not have done that surgery with a correction as much as I needed.)

Anyway, eyesight can be corrected to better than 20/20, although that means that the light is exactly focused on the retina. (In myopia, it is focused behind the retina and in hyperopia [far-sightedness] it is focused in front of the retina. I'm not quite sure how that's possible. There were previous threads on this which you can look up, and it was stated there that 20/20 is just an average of good sight. However, it does mean that the light is focused on the retina, inspite of what others may have posted. I believe though it may be just an approximation of being focused on the retina and it's possible to be even more focused, or it's possible the number of rods and cones may have something to do with it.

10-27-2001, 07:48 PM
In this thread (http://boards.straightdope.com/sdmb/showthread.php?threadid=55629) scoth said that better than 20/20 is possible due to the smoothness of the cornea. He talks about mapping the cornea in a special type of laser; however, before any laser is done, the cornea is mapped topographically. I had it done before my laser surgery that never was.