# What metric lingo is used in describing eye defects? **URL:** \ **Category:** Factual Questions\ **Created:** [January 29, 2016, 6:00am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555 "2016-01-29T06:00:30Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![davidmich](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@davidmich](https://boards.straightdope.com/u/davidmich)\ **Post date:** [January 29, 2016, 6:00am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/1 "2016-01-29T06:00:30Z")
Hi What metric lingo is used in describing eye defects? If someone has a 0.8 right eye and 1.0 left eye, what metric terms are used after the numbers themselves? I look forward to your feedback. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [January 29, 2016, 6:02am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/2 "2016-01-29T06:02:29Z")
“Metric” lingo? The unit you appear to be using is diopters, but I don’t know if they’re part of the IS or a unit of their own. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [January 29, 2016, 6:11am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/3 "2016-01-29T06:11:52Z")
I don’t understand your units. Are you referring to an eyeglass/contact prescription? The standard measurement for that is diopters/dioptres, which is a metric unit. It is the reciprocal of the focal length in meters. AFAIK there is not an equivalent that uses non-SI units. Technically though it isn’t part of the SI standard, and the general term is “inverse meter.” 0.8 and 1.0 are meaningless without a + or - before it. The minus indicates myopia aka nearsightness, or an inability to see far. The positive is hyperopia or farsightedness, and requires reading glasses. Eyeglass prescriptions may be followed by things like SPH or CYL, but those aren’t units but designate the symmetry of the lens. ---
**Author:** ![davidmich](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@davidmich](https://boards.straightdope.com/u/davidmich)\ **Post date:** [January 29, 2016, 6:28am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/4 "2016-01-29T06:28:17Z")
> [@thelurkinghorror](#): > > I don’t understand your units. Are you referring to an eyeglass/contact prescription? The standard measurement for that is diopters/dioptres, which is a metric unit. It is the reciprocal of the focal length in meters. AFAIK there is not an equivalent that uses non-SI units. Technically though it isn’t part of the SI standard, and the general term is “inverse meter.” > > 0.8 and 1.0 are meaningless without a + or - before it. The minus indicates myopia aka nearsightness, or an inability to see far. The positive is hyperopia or farsightedness, and requires reading glasses. Eyeglass prescriptions may be followed by things like SPH or CYL, but those aren’t units but designate the symmetry of the lens. Thanks thelurking horror. Thanks for clarifying the terminology. So if I understand you correctly, if there were a + and \_ after the figures, they would simply be followed by the word diopters or in abbreviated form(d). But I’m not asking about metric for prescription purposes. I’m looking for any metrics following figures that indicate vision power. ---
**Author:** ![davidmich](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@davidmich](https://boards.straightdope.com/u/davidmich)\ **Post date:** [January 29, 2016, 6:54am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/5 "2016-01-29T06:54:53Z")
Ok. I understand that the decimals are measurements of diopter. They are for prescription Thanks all. > **[Live Your Best Life - Living Well With Low Vision](https://lowvision.preventblindness.org)** > > The mission of Living Well With Low Vision is to make it as easy as possible for people to educate themselves about loss of vision and to meet the daily Prescriptions for eyeglasses are measured in diopters. The measurement starts at zero (“plano”), with four quarters to a diopter: 0.25 (a quarter diopter), 0.50 (one half a diopter), 0.75 (3/4 of a diopter), and 1.00 (one diopter). The higher the numbers, the higher (stronger) the prescription. Here are estimates for the approximate correction needed for each line seen on the Snellen chart: Snellen 20/10 20/15 20/20 20/30 20/40 20/50 20/100 20/200 Estimated prescription Plano (zero) Plano Plano to -0.25 -0.50 -0.75 -1.00 to -1.25 -1.75 to -2.00 -2.00 to -2.50 ---
**Author:** ![Mops](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mops/32/16494_2.png) [@Mops](https://boards.straightdope.com/u/Mops)\ **Post date:** [January 29, 2016, 7:02am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/6 "2016-01-29T07:02:04Z")
If you are talking about visual acuity rather than focus length, in metric countries it’s expressed either as a fraction with higher numerator and denominator but the same value, or as a number, or as a percentage. For example, the US 20/20 would be either 6/6, or 1.00, or 100 %. The latter is commonly used in Germany for example - a patient who is told that he has 20/25 vision in the US, gets told he has 80% vision in Germany. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [January 29, 2016, 7:55am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/7 "2016-01-29T07:55:57Z")
That’s misleading terminology, good job Germany. So I’ve got about 4% vision according to that. Looks like the gubmint owes me retroactive deductions for being blind. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [January 29, 2016, 8:49am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/8 "2016-01-29T08:49:08Z")
> [@Nava](#): > > “Metric” lingo? The unit you appear to be using is diopters, but I don’t know if they’re part of the IS or a unit of their own. The proper SI unit name would be the inverse metre (m[sup]-1[/sup]), but it’s equivalent to the dioptre anyway. ---
**Author:** ![Donnerwetter](https://avatars.discourse-cdn.com/v4/letter/d/b2d939/32.png) [@Donnerwetter](https://boards.straightdope.com/u/Donnerwetter)\ **Post date:** [January 29, 2016, 11:47am UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/9 "2016-01-29T11:47:09Z")
> [@thelurkinghorror](#): > > That’s misleading terminology, good job Germany. So I’ve got about 4% vision according to that. Looks like the gubmint owes me retroactive deductions for being blind. I guess _uncorrected vision acuity_ is the issue here. If yours is 4 %, I assume that you very much rely upon your glasses, but if you have them on, you’re fine and fully functional (including driving a car etc.). ---
**Author:** ![davidmich](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@davidmich](https://boards.straightdope.com/u/davidmich)\ **Post date:** [January 29, 2016, 2:19pm UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/10 "2016-01-29T14:19:44Z")
> [@MrDibble](#): > > The proper SI unit name would be the inverse metre (m[sup]-1[/sup]), but it’s equivalent to the dioptre anyway. Thanks Mr. Dibble. Thank you all. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 29, 2016, 2:34pm UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/11 "2016-01-29T14:34:58Z")
Diopters are used because they measure _optical power_, rather than _focal length_. The optical power is the inverse of the focal length. The reason is that, if you place two lenses next to each other with negligible spacing, the optical power of the combination is simply the sum of theuir individual powers. 0.5 D + 0.5 D = 1 D Focal lengths, on the other hand, don’t add, so it’s less obvious that the result of putting a 2 meter focal length lens together with a 2w meter focal length lens is a one meter focal length lens – but that’s exactly the same as the above example with diopters. You can see the point of doing this when you consider that using eyeglasses you add the power of the glasses to that of your eye. Incidentally, if you’re astigmatic (like me) your prescription has three numbers for each eye. An astigmatic eye has different focal lengths along different axes. Instead of giving the power along each axis, the astigmatic prescription gives a base power that holds for all directions, an astigmatic power that is added along one rotational axis, and the angle of that rotation. ---
**Author:** ![Donnerwetter](https://avatars.discourse-cdn.com/v4/letter/d/b2d939/32.png) [@Donnerwetter](https://boards.straightdope.com/u/Donnerwetter)\ **Post date:** [January 29, 2016, 2:56pm UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/12 "2016-01-29T14:56:18Z")
> [@CalMeacham](#): > > Incidentally, if you’re astigmatic (like me) your prescription has three numbers for each eye. An astigmatic eye has different focal lengths along different axes. Instead of giving the power along each axis, the astigmatic prescription gives a base power that holds for all directions, an astigmatic power that is added along one rotational axis, and the angle of that rotation. And if you’re getting older, there is one more number, also given in diopters, indicating the additional optical power needed for reading. This is my prescription: Right Eye Left Eye Sphere +1.00 dpt +3.25 dpt Cylinder 0.50 dpt 0.50 dpt Axis 5° 47° Addition +2.00 dpt +2.00 dpt ---
**Author:** ![jtur88](https://avatars.discourse-cdn.com/v4/letter/j/e9c0ed/32.png) [@jtur88](https://boards.straightdope.com/u/jtur88)\ **Post date:** [January 29, 2016, 4:15pm UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/13 "2016-01-29T16:15:02Z")
> [@thelurkinghorror](#): > > That’s misleading terminology, good job Germany. So I’ve got about 4% vision according to that. Looks like the gubmint owes me retroactive deductions for being blind. Legal blindness is for uncorrectable vision deficiency. If your poor vision can be corrected with lenses, you are not blind. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [January 29, 2016, 5:53pm UTC](https://boards.straightdope.com/t/what-metric-lingo-is-used-in-describing-eye-defects/744555/14 "2016-01-29T17:53:52Z")
> [@jtur88](#): > > Legal blindness is for uncorrectable vision deficiency. If your poor vision can be corrected with lenses, you are not blind. I know, it was a joke on their terminology.