# Measuring Eyesight **URL:** \ **Category:** Factual Questions\ **Created:** [April 18, 2007, 7:53am UTC](https://boards.straightdope.com/t/measuring-eyesight/400594 "2007-04-18T07:53:41Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![MistWing](https://avatars.discourse-cdn.com/v4/letter/m/ccd318/32.png) [@MistWing](https://boards.straightdope.com/u/MistWing)\ **Post date:** [April 18, 2007, 7:53am UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/1 "2007-04-18T07:53:41Z")
When you go to an optometrist to get your eyesight checked, you can get something like 20/40 (indicating near-sightedness), 20/20 (indicating ‘perfect vision’), or 20/5 (indicating better than ‘perfect vision’). How is far-sightedness indicated? MistWing SilverTail ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 18, 2007, 11:52am UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/2 "2007-04-18T11:52:00Z")
[QUOTE=MistWing] When you go to an optometrist to get your eyesight checked, you can get something like 20/40 (indicating near-sightedness), 20/20 (indicating ‘perfect vision’), or 20/5 (indicating better than ‘perfect vision’). How is far-sightedness indicated? MistWing SilverTail [/QUOTE] The degree of both myopia and hyperopia are best expressed by the prescription required to treat them. The strength of a corrective lens is measured in diopters, with a positive sign for treating far sight, and a negative sign for treating short sight. > **[Dioptre](https://en.wikipedia.org/wiki/Dioptre)** > > A dioptre (British spelling) or diopter (American spelling), symbol dpt or D, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dpt = 1 m−1. It is normally used to express the optical power of a lens or curved mirror, which is a physical quantity equal to the reciprocal of the focal length, expressed in metres. For example, a 3-dioptre lens brings parallel rays of light to focus at .mw-parser-output .frac{white-space:nowrap}.mw-parser-output .fra... ---
**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:** [April 18, 2007, 11:56am UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/3 "2007-04-18T11:56:05Z")
I don’t know if there’s any wa to put it into “20/whatever” shorthand, but, properly, optometrists quantify errors in your vision focus in terms of _Diopters_, which are measures of what optical _power_ needs to be added to your eye’s own focusing ability to turn it into normal vision. “Power” is the inverse of the focal length defect measured in meters. If you have positive error of 2 Diopters, you’re nearsighted (myopic) and you need a -2D lens (that is, a negative lens with a focal length of 1/2 meter, or 50 cm) to fix it. Negative lenses are the kind that make things seem smaller. They spread light out, and don’t form an image. If you are presbyopic or farsighted, you’ll have a negative error and need a positive lens to correct it. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [April 18, 2007, 4:13pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/4 "2007-04-18T16:13:21Z")
[QUOTE=MistWing] When you go to an optometrist to get your eyesight checked, you can get something like 20/40 (indicating near-sightedness), 20/20 (indicating ‘perfect vision’), or 20/5 (indicating better than ‘perfect vision’). How is far-sightedness indicated? [/QUOTE] By something like 20/5. It doesn’t mean “better than perfect” vision. 20/20 means you see at 20 yards with the clarity that “normal” eyes see at 20 yards. 20/40 means you see at 20 yards with the clarity normally seen at 40 yards - in other words, not so clearly. Since you (presumably) see near things clearly, but not far things, you are nearsighted. 20/5 means you see at 20 yards with the clarity normally seen at 5 yards. Since you see far things more clearly than average, you are farsighted. Your near vision might suck, though. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 18, 2007, 4:24pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/5 "2007-04-18T16:24:52Z")
[QUOTE=Gary T] By something like 20/5. It doesn’t mean “better than perfect” vision. 20/20 means you see at 20 yards with the clarity that “normal” eyes see at 20 yards. 20/40 means you see at 20 yards with the clarity normally seen at 40 yards - in other words, not so clearly. Since you (presumably) see near things clearly, but not far things, you are nearsighted. 20/5 means you see at 20 yards with the clarity normally seen at 5 yards. Since you see far things more clearly than average, you are farsighted. Your near vision might suck, though. [/QUOTE] Wrong. 20/5 indicates exceptional distance vision, and implies absolutely nothing about near vision. It’s a crude measure of visual acuity, and means very little in terms of diagnosis or correction, just as “farsighted” is a crude and archaic term that says very little about what’s actually happening. 20/20 indicates average resolution, by definition, at 20 feet. Mild hyperopes often have no difficulty at this distance, but severe hyperopia will cause problems at all distances. Properly corrected, any person might still have only 20/40 acuity due to other problems. ---
**Author:** ![Nars\_Glinley](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nars_glinley/32/8177_2.png) [@Nars\_Glinley](https://boards.straightdope.com/u/Nars_Glinley)\ **Post date:** [April 18, 2007, 5:51pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/6 "2007-04-18T17:51:17Z")
[QUOTE=MistWing] When you go to an optometrist to get your eyesight checked, you can get something like 20/40 (indicating near-sightedness), 20/20 (indicating ‘perfect vision’), or 20/5 (indicating better than ‘perfect vision’). [/QUOTE] I’ve been going to the eye doctor since first grade (38 years ago) and only once did the doctor describe my vision in terms of 20/something. The only reason that he did was because I asked him to. He estimated that mine was 20/600. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [April 18, 2007, 6:06pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/7 "2007-04-18T18:06:51Z")
[QUOTE=Nametag] Wrong. 20/5 indicates exceptional distance vision, and implies absolutely nothing about near vision. [/QUOTE] Wrong? How so? Is not “see far things more clearly than average” roughly equivalent to “exceptional distance vision”? Does not “near vision might suck” - please notice the word MIGHT - indicate that nothing is implied about near vision? ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 19, 2007, 1:44pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/8 "2007-04-19T13:44:11Z")
[QUOTE=Gary T] Wrong? How so? Is not “see far things more clearly than average” roughly equivalent to “exceptional distance vision”? Does not “near vision might suck” - please notice the word MIGHT - indicate that nothing is implied about near vision? [/QUOTE] > [@Gary T somewhat earlier](#): > > [Farsightedness would be indicated] **By something like 20/5**. It doesn’t mean “better than perfect” vision. > > 20/5 means you see at 20 yards with the clarity normally seen at 5 yards. **Since you see far things more clearly than average, you are farsighted.** Your near vision might suck, though. The bolded statements are directly contrary to fact. Just because other statements weren’t, doesn’t mean the answer as a whole wasn’t still wrong. Farsighted people, on average, do not test at better than 20/20. People who do test better than 20/20 generally have no optical defects at all. And the Snell chart (basis for “20/xx”) is read at 20 feet, not 20 yards. ---
**Author:** ![rowrrbazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rowrrbazzle/32/414_2.png) [@rowrrbazzle](https://boards.straightdope.com/u/rowrrbazzle)\ **Post date:** [April 20, 2007, 8:19pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/9 "2007-04-20T20:19:05Z")
If you wanted to extend the 20/20 method to farsightedness, I think one way to do that would be to decrease the first number, like 10/20 - what most people can see at 10 feet, you have to be 20 feet away to see clearly. However, there are at least two problems - 1. You have to give up keeping the first number constant as a standard should be. 2. I’m guessing farsightedness is very rarely as extreme as that. ---
**Author:** ![nebco9](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@nebco9](https://boards.straightdope.com/u/nebco9)\ **Post date:** [April 20, 2007, 8:54pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/10 "2007-04-20T20:54:20Z")
[QUOTE=KRM] I’ve been going to the eye doctor since first grade (38 years ago) and only once did the doctor describe my vision in terms of 20/something. The only reason that he did was because I asked him to. He estimated that mine was 20/600. [/QUOTE] I hope that you must have either misunderstood the doctor, or else you meant to type 20/60. 20/20 is considered normal vision. At 20/200, you can be considered [“legally blind”](http://en.wikipedia.org/wiki/Legally_blind). At 20/600?..Well, let’s just say that’s really, really bad. :eek: ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 20, 2007, 10:23pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/11 "2007-04-20T22:23:35Z")
[QUOTE=nebco9] I hope that you must have either misunderstood the doctor, or else you meant to type 20/60. 20/20 is considered normal vision. At 20/200, you can be considered [“legally blind”](http://en.wikipedia.org/wiki/Legally_blind). At 20/600?..Well, let’s just say that’s really, really bad. :eek: [/QUOTE] 20/200 is “legally blind” if that’s your best _corrected_ acuity. 20/600 translates to roughly -5.00 diopters (for myopia), and both lenses of my prescription are stronger than that. It’s poor vision, but nothing like post-cataract vision. If your vision is correctable to 20/20 with glasses or contacts, that’s the only number that counts. ---
**Author:** ![MovieMogul](https://avatars.discourse-cdn.com/v4/letter/m/f08c70/32.png) [@MovieMogul](https://boards.straightdope.com/u/MovieMogul)\ **Post date:** [April 20, 2007, 10:42pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/12 "2007-04-20T22:42:47Z")
I don’t know what “legally blind” signifies in any meaningful sense, because even though I’m “off the charts” so-to-speak, as long as your vision is correctible, you’re not “legally” anything. This is according to my eye doctor who, in response to my “What my 20/X vision?” question, put up the BIG E and asked “Can you see that Big E?” I couldn’t (except as a big fuzzy black spider thingy), so I was “off the chart”. [hj]Whoop! I ordered my new glasses today! Makes no real difference to me, but Wifey likes the narrow, frameless, sexy ones so that’s what I got.[/hj] ---
**Author:** ![Nars\_Glinley](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nars_glinley/32/8177_2.png) [@Nars\_Glinley](https://boards.straightdope.com/u/Nars_Glinley)\ **Post date:** [April 21, 2007, 2:05am UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/13 "2007-04-21T02:05:51Z")
[QUOTE=nebco9] I hope that you must have either misunderstood the doctor, or else you meant to type 20/60. 20/20 is considered normal vision. At 20/200, you can be considered [“legally blind”](http://en.wikipedia.org/wiki/Legally_blind). At 20/600?..Well, let’s just say that’s really, really bad. :eek: [/QUOTE] I have bad eyes. According to **Nametag** , it’s even worse than I thought. I don’t remember my exact prescription but I know that both eyes require at least 8 diopters of correction. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 21, 2007, 12:06pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/14 "2007-04-21T12:06:56Z")
[QUOTE=KRM] I have bad eyes. According to **Nametag** , it’s even worse than I thought. I don’t remember my exact prescription but I know that both eyes require at least 8 diopters of correction. [/QUOTE] Yeah, you’re working up to 20/1000 territory. Cataract lenses can be as strong as 30 - 40 diopters – at that point, 20/whatever is pretty meaningless. There’s no “normal vision” comparison of what a post-cataract patient can (well, can’t) see at 20 feet. ---
**Author:** ![Ferret\_Herder](https://avatars.discourse-cdn.com/v4/letter/f/e47774/32.png) [@Ferret\_Herder](https://boards.straightdope.com/u/Ferret_Herder)\ **Post date:** [April 21, 2007, 12:53pm UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/15 "2007-04-21T12:53:57Z")
[QUOTE=ArchiveGuy] I don’t know what “legally blind” signifies in any meaningful sense, because even though I’m “off the charts” so-to-speak, as long as your vision is correctible, you’re not “legally” anything. [/QUOTE] Yes, the Wiki link quoted above regarding being “legally blind” specifically states that you are only if that’s your **best corrected** vision. I work in an ophthalmology department, and at least one of our retinal specialist doctors will confuse the hell out of patients by complimenting their (best-corrected) vision when it’s 20/20 or better. Of course, they’re thinking about how strong their glasses are, but in comparison to patients he sees who are often “legally blind” at best-corrected, or at the “count fingers at 1 foot away” or worse level, having to wear glasses to get perfect vision isn’t seen as that big of a deal by a lot of ophthalmologists. The Snellen fraction (“20/whatever”) is a rough shorthand way of describing vision, but isn’t nearly as useful as the “prescription” from a refraction. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [April 22, 2007, 12:14am UTC](https://boards.straightdope.com/t/measuring-eyesight/400594/16 "2007-04-22T00:14:23Z")
I should mention that there are two broad areas of poor vision. The first area is the largely correctable “refractive error” – myopia, hyperopia, astigmatism, and presbyopia. All of these can be treated with glasses, contacts, laser surgery, or some combination thereof. The second area is “low vision”; this catch-all term includes cataracts, degenerative eye diseases, and eye injuries. While there are medical or surgical treatments that can reverse or or halt some of the damage from some of these conditions, low vision patients are generally stuck with an uncorrectable loss of acuity that can only be coped with. From these ranks come the “legally blind”; no matter what kind of glasses they wear, their acuity will never be better than 20/200, and their lives are spent working around the disability or restricting their activities.