diopters vs "20/somthing"

[xanthines]

I was wondering if there was a correlation between corrective eyewear prescriptions (diopters) and the Snellen(?) vision test (you know, 20/20, 20/80, 20/something). So my contacts are -2.25 diopters which would I have 20/what vision? I don't remember, but I think my doc said 20/100 or 20/200.

There's no real valid reason I'm asking this. I just think of weird questions sometimes. The effects of not having cable TV, I suppose.

-X

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[sco1styear]

A VERY rough estimate is to correlate 20/100 with -1.00D 20/200 with -2.00D etc. That's the easiest way to remember it...
To be more accurate it's more along the lines of:
20/30 0.50
20/40 0.75
20/50 1.00
20/70 1.25
20/100 1.50
20/150 2.00
20/200 2.50
20/250 3.00
20/300 3.50
20/400 4.50
But this is different if you have astigmatism

 

[rpames]

This rule-of-thumb becomes more inaccurate the high the refractive error.

 

[UABopt]

I was wondering if there was a correlation between corrective eyewear prescriptions (diopters) and the Snellen(?) vision test (you know, 20/20, 20/80, 20/something). So my contacts are -2.25 diopters which would I have 20/what vision? I don't remember, but I think my doc said 20/100 or 20/200.

There's no real valid reason I'm asking this. I just think of weird questions sometimes. The effects of not having cable TV, I suppose.

-X

Remember though, while sometimes handy, those are only very rough estimates. Refractive error and visual acuity are two separate and distinct things, and while related, it could never be possible to determine one from the other with any degree of certainty. While a “standard observer” can resolve 1 minute of arc (snellen acuity of 20/20), saying a person has a VA of 20/20 is far from being equivalent to saying this person has perfect vision, in fact the average emetropic (corrected or otherwise) person has a VA of 20/16 or so .
Some people cannot be refracted to 20/20 or beyond. It is thus possible, for example, to have a person with only 1.00D (or less) refractive error, but still have a best corrected VA of 20/100 or worse. In such a case, a poor VA could be indicative of some other underlying pathology.

 

[scott McGregor]

I was wondering if there was a correlation between corrective eyewear prescriptions (diopters) and the Snellen(?) vision test (you know, 20/20, 20/80, 20/something). So my contacts are -2.25 diopters which would I have 20/what vision? I don't remember, but I think my doc said 20/100 or 20/200.

There's no real valid reason I'm asking this. I just think of weird questions sometimes. The effects of not having cable TV, I suppose.

-X

While it is only an estimate, E. Borish published such a table in his multi volume text "Clinical Refraction". It is a chart/grid that has spherical power across the top and astigmatic (cylinder) power down the side of the grid. Join the two columns out on the grid and it assigns an acuity to that prescription.

 

[Twiigg]

BUMP!

I have -4.25 diopter lenses. What is this approximately in 20/x? If this is still unanswerable, then would I likely be higher 20/1000 or 20/200 taking the Snelling test w/o any corrective eye-wear?

I'm really stressed on figuring this out because I have always considered different paths in the military and they all use 20/x scale when listing vision requirements! Please help!

Finally, if it is just impossible to tell, then is there anyway I could perform this test on my own at home? I asked my optometrist and he said he didn't know how to test my vision on the 20/x scale. Hopefully some of you are better informed!

Thanks a ton!

 

[JMU07]

BUMP!

I have -4.25 diopter lenses. What is this approximately in 20/x? If this is still unanswerable, then would I likely be higher 20/1000 or 20/200 taking the Snelling test w/o any corrective eye-wear?

I'm really stressed on figuring this out because I have always considered different paths in the military and they all use 20/x scale when listing vision requirements! Please help!

Finally, if it is just impossible to tell, then is there anyway I could perform this test on my own at home? I asked my optometrist and he said he didn't know how to test my vision on the 20/x scale. Hopefully some of you are better informed!

Thanks a ton!

There is no formula to get one number from the other. The estimates given above are VERY rough estimates, so using that a -4.00 would be close to 20/400 ( but that, again, is NOT 100% accurate). I do not understand how your OD does not know how to tell you what your acuities are, as it is required to be recorded at every eye exam.

 

[Twiigg]

There is no formula to get one number from the other. The estimates given above are VERY rough estimates, so using that a -4.00 would be close to 20/400 ( but that, again, is NOT 100% accurate). I do not understand how your OD does not know how to tell you what your acuities are, as it is required to be recorded at every eye exam.

Well, he just always records it in the -4.00 etc. scale.

 

[JMU07]

Well, he just always records it in the -4.00 etc. scale.

Right. By law, ODs are supposed to record your Rx and your acuities. (I know this is true in most states, but I'm not sure if it's a national thing.)