How do I convert these numbers?

Question

9/12 to for example -2.5 or .....etc. when reading optometrist reports, is there a chart or a website the may help

Answer 1

The only answer I have is that if it reads -2.50 is that you are nearsighted. More info, please.

Answer 2

Visual acuity is determined by checking with an eye chart.

The strength of lenses is determined by measurement.

Answer 3

the 9/12 mean at a normal person see object at 12 m you see it at 9 m

but -2.5 is the power of your lens to correct this refractive error

Answer 4

i'm with coffee...

those are 2 totally different things.  the "9/12" is a (weird) measurement of visual acuity.  it doesnt tell you anything about what Rx would make your daughter see well.  those are not that related.

so the answer to the question of "how do i convert these numbers?" is...you cant.  there is no conversion.  apples & oranges.

Answer 5

You can do it using a formula but you need measurements of your son's eyeball to do it properly ... just easier to ask the optician what strength lenses are required, to be quite honest! Sorry.

Answer 6

There is a table that can give you an approximation.  Acuity really depends on the patient, though, as others have said.  Anyways, here's a chart:

20/20 = 0.00

20/25 = 0.25

20/30 = 0.50

20/40 = 0.75

20/50 = 1.00

20/70 = 1.25

20/100 = 1.50

20/200 = 2.50

Remember, these are approximations and don't account for astigmatism, but maybe it will help a little.  And I'm confused about the 9/12 number, too.  Is that some alternative metric measurement or something?