Eye strengths in numbers

Question

What are the numbered eye strengths that you see

1.25

-1.10

500/600

etc

is there a list of them?

whats the highest and the lowest?

Answer 1

Distance vision is usually given as 20/20.  This means that you can seen at 20 feet what a normal eye sees at 20 feet.  20/50 would mean that what a normal eye sees at 50 feet, you see at 20.  It's poor vision.  You have to get closer to see it.   On the other hand, if you have vision of 20/15, it means that what a normal eye sees at 15 feet, you see at 20--vision good enough for a fighter pilot.  I believe this is an open-ended scale of vision.  I had vision of 20/400 before surgery.

There also are measurements for astigmatism, but I am not sure how those are read.

Answer 2

Prescriptions are given in diopters. Minus numbers are for nearsighted eyes, and make things smaller. Plus numbers are for farsighted eyes and make things bigger.

A prescription of -2.00 is twice the strength of a prescription of -1.00, and so on. The lowest is -0.25, and usually the increases are a quarter of a diopter. Theoretically there is no upper limit, although there comes a point where the correction just doesn't work due to distortion etc.

Being nearsighted is different to being farsighted, in several ways. It isn't just the "opposite" error. A person who needs -2.00 will cope without glasses if they have to, but will have problems with pretty much everything. A young person who needs +2.00 can probably manage just fine, due to the accomodation ("zoom') of the lens of their eyes. However, an older person might be quite dependant on their +1.50 glasses.

-1.00 is about the point at which you need glasses, and would probably require them to drive safely. The point at which people wear their glasses all day varies from person to person, some people tolerate the blur better than others, but most people don't tolerate it well much after -2.00.

The point at which you are de facto helpless without glasses is about -5.00, but I have personally met people with prescriptions of -28.00, and I'm sure the professionals that visit this site have come across higher.

There are other parts of the prescription such as cylinder for astigmatism which are more complex, and require two figures, the diopter and the direction of the cylinder, given in degrees.

20/20 and so on is explained by another answerer. It corresponds approximately to the prescription, but not always. As a rough guide -1.00 is 20/40. 20/1000 is about -7.50.