# Can you tell your 20/20 eye acuity from your glasses prescription? **URL:** \ **Category:** Factual Questions\ **Created:** [October 10, 2009, 8:02pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336 "2009-10-10T20:02:41Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![TheMadHun](https://avatars.discourse-cdn.com/v4/letter/t/53a042/32.png) [@TheMadHun](https://boards.straightdope.com/u/TheMadHun)\ **Post date:** [October 10, 2009, 8:02pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/1 "2009-10-10T20:02:41Z")
If my prescription is -4.0, what would my “Snellen chart” number be? You know, that chart that goes from 20/200 for the big E down to 20/20 for average and even lower. (Lower is for testing eagles and zoom lenses? Unclear, Captain.) Makes me wonder if these things are different in metric. But that’s secondary. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [October 10, 2009, 8:22pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/2 "2009-10-10T20:22:35Z")
Eye charts you can print out [http://www.i-see.org/eyecharts.html](http://www.i-see.org/eyecharts.html) ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [October 10, 2009, 10:42pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/3 "2009-10-10T22:42:26Z")
My ophthalmologist has assured me that you can’t take a lens prescription and tell how good someone’s uncorrected vision is. He says there are too many other factors, including interpolation by the brain. This conversation has actually come up with multiple doctors, who are all surprised I see as well as I do with eyes as screwed up as I have. ---
**Author:** ![Claude\_Remains](https://avatars.discourse-cdn.com/v4/letter/c/898d66/32.png) [@Claude\_Remains](https://boards.straightdope.com/u/Claude_Remains)\ **Post date:** [October 11, 2009, 12:11am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/4 "2009-10-11T00:11:30Z")
Interpolation. Very interesting, thank you. ---
**Author:** ![Alex\_Dubinsky](https://avatars.discourse-cdn.com/v4/letter/a/dbc845/32.png) [@Alex\_Dubinsky](https://boards.straightdope.com/u/Alex_Dubinsky)\ **Post date:** [October 11, 2009, 12:22am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/5 "2009-10-11T00:22:01Z")
Your eyes have a sweet spot of comfort, and a larger range where they still function. Glasses prescriptions target the sweet spot, while eye charts measure the fringes. (As I side-note, I feel that when glasses target the sweet spot they do a disservice to vision. Operating at the fringes keeps them from closing in.) ---
**Author:** ![cromulent](https://avatars.discourse-cdn.com/v4/letter/c/b4bc9f/32.png) [@cromulent](https://boards.straightdope.com/u/cromulent)\ **Post date:** [October 11, 2009, 2:27pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/6 "2009-10-11T14:27:48Z")
You can estimate the visual acuity for a given level of myopia (nearsightedness) in diopters, assuming no other ocular problems/disease. Farsightedness (hyperopia) can be “focused through” assuming the patient has enough focusing ability to counteract the hyperopia, except for relatively high prescriptions. The amount of focusing ability in diopters that a patient has is dependent on age, so the VA of a hyperope is a function that depends on the two variables of a) refractive error and b) accommodative (focusing) ability. The level of astigmatism in diopters can also be used to estimate VA, again assuming no other ocular disease is present. Depending on the axis of the astigmatism, people have a greater/lesser ability to squint to clear the incoming blurry image, which is why some angles of astigmatic error are more likely to lead to the development of amblyopia in children than others. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [October 12, 2009, 9:55pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/7 "2009-10-12T21:55:19Z")
> [@TheMadHun](#): > > If my prescription is -4.0, what would my “Snellen chart” number be? > You know, that chart that goes from 20/200 for the big E down to 20/20 for average and even lower. (Lower is for testing eagles and zoom lenses? Unclear, Captain.) > > Makes me wonder if these things are different in metric. But that’s secondary. At -4 diopters, your vision is off the chart. The Snellen chart goes only to 20/200, which is legal blindness if uncorrectable. That would correlate to a diopter of about -2, but that’s only an approximation. As other posters noted, there are variables. ---
**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:** [October 12, 2009, 10:51pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/8 "2009-10-12T22:51:44Z")
> [@barbitu8](#): > > At -4 diopters, your vision is off the chart. The Snellen chart goes only to 20/200, which is legal blindness if uncorrectable. That would correlate to a diopter of about -2, but that’s only an approximation. As other posters noted, there are variables. You can move an eye chart closer and get a larger Snellen fraction, but I can’t come up with an approximate diopter off the top of my head for that. ---
**Author:** ![Donovan](https://avatars.discourse-cdn.com/v4/letter/d/ea666f/32.png) [@Donovan](https://boards.straightdope.com/u/Donovan)\ **Post date:** [October 13, 2009, 1:08am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/9 "2009-10-13T01:08:32Z")
> [@TheMadHun](#): > > (Lower is for testing eagles and zoom lenses? Unclear, Captain.) After we took eye exams when we were kids, my sis used to brag that she had 20/10 vision, which she claimed meant she had better vision than normal. Now that I think about it, she might have just been bullshitting, anyone want to say whether people would get a ‘better than 20/20’ rating, or even if it is possible? ---
**Author:** ![Monty](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/monty/32/47_2.png) [@Monty](https://boards.straightdope.com/u/Monty)\ **Post date:** [October 13, 2009, 1:29am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/10 "2009-10-13T01:29:05Z")
Wouldn’t the actual distance the retina is from the eye’s lens be important to know to answer the question? ---
**Author:** ![cromulent](https://avatars.discourse-cdn.com/v4/letter/c/b4bc9f/32.png) [@cromulent](https://boards.straightdope.com/u/cromulent)\ **Post date:** [October 13, 2009, 1:38am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/11 "2009-10-13T01:38:22Z")
> [@Donovan](#): > > After we took eye exams when we were kids, my sis used to brag that she had 20/10 vision, which she claimed meant she had better vision than normal. > > Now that I think about it, she might have just been bullshitting, anyone want to say whether people would get a ‘better than 20/20’ rating, or even if it is possible? It’s definitely possible to get down below 20/20 acuity; in fact i’d wager a guess that most young people who can achieve 20/20 as their best corrected or uncorrected acuity could also achieve 20/15, or at least most of the 20/15 letters on the chart, if the eye doctor bothers to test down that far (and most don’t.) 20/10 acuity is less common, but certainly possible. ---
**Author:** ![groman](https://avatars.discourse-cdn.com/v4/letter/g/73ab20/32.png) [@groman](https://boards.straightdope.com/u/groman)\ **Post date:** [October 13, 2009, 3:12am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/12 "2009-10-13T03:12:56Z")
> [@Donovan](#): > > Now that I think about it, she might have just been bullshitting, anyone want to say whether people would get a ‘better than 20/20’ rating, or even if it is possible? I know I have better than 20/15 vision, but I am not sure how much better, and I’m in my mid-twenties. It’s most certainly possible (if not common?) for a child to have 20/10. ---
**Author:** ![Alex\_Dubinsky](https://avatars.discourse-cdn.com/v4/letter/a/dbc845/32.png) [@Alex\_Dubinsky](https://boards.straightdope.com/u/Alex_Dubinsky)\ **Post date:** [October 13, 2009, 4:11am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/13 "2009-10-13T04:11:30Z")
Isn’t 20/20 just average? About HALF of people exceed it. What the numbers mean is that at 20 feet away you can read things others can typically also read at 20’. With 20/10, you can read things from 20’ someone else would have to be 10’ away from. (20/200 means that you can’t even see letters that are normally visible from 60 yards.) Does anyone know what the record for vision has been? I mean, how far above average can you be? ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [October 13, 2009, 7:43pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/14 "2009-10-13T19:43:11Z")
20/20 is not just an average. It means that the light entering the cornea is focused by the lens on the retina, not short of it (near-sight or myopia) or beyond it (far-sight or hyperopia). (Presbyopia is sometimes called far-sightedness, but this refers to the “old-age” vision.) Since 20/20 means exact focus, I’ve always wondered how some people have better than 20/20. Many pilots, for example, have 20/10. Somebody told me it is because they have more rods in their retina which gives them the ability to see better. I’ve heard no better explanation. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [October 13, 2009, 8:17pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/15 "2009-10-13T20:17:47Z")
> [@barbitu8](#): > > 20/20 is not just an average. It means that the light entering the cornea is focused by the lens on the retina, not short of it (near-sight or myopia) or beyond it (far-sight or hyperopia). (Presbyopia is sometimes called far-sightedness, but this refers to the “old-age” vision.) Since 20/20 means exact focus, I’ve always wondered how some people have better than 20/20. Many pilots, for example, have 20/10. Somebody told me it is because they have more rods in their retina which gives them the ability to see better. I’ve heard no better explanation. Unfortunately, that’s not correct. 20/20 is defined as “normal vision” and the other fractions represent how far off from that you are. Here’s some more detail: [http://www.howstuffworks.com/question126.htm](http://www.howstuffworks.com/question126.htm) The number of rods and cones might have something to do with better vision, but there are a lot of factors, including the shape of the cornea, the strength of the muscles controlling the lens and the flexibility of the lens itself. When you’re young, those muscles are strong and the lens is very soft. As you age, the lens gets tougher and the muscles weaker, hence the typical vision problems that set in. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [October 13, 2009, 8:57pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/16 "2009-10-13T20:57:53Z")
> [@dracoi](#): > > Unfortunately, that’s not correct. 20/20 is defined as “normal vision” and the other fractions represent how far off from that you are. Here’s some more detail: [http://www.howstuffworks.com/question126.htm](http://www.howstuffworks.com/question126.htm) > > The number of rods and cones might have something to do with better vision, but there are a lot of factors, including the shape of the cornea, the strength of the muscles controlling the lens and the flexibility of the lens itself. When you’re young, those muscles are strong and the lens is very soft. As you age, the lens gets tougher and the muscles weaker, hence the typical vision problems that set in. I stand by my statement. Your link does not describe the physiology of vision. Try [Visual acuity - Wikipedia](http://en.wikipedia.org/wiki/Visual_acuity#.22Normal.22_vision) The shape of the cornea and the length of the eyeball determine where the focused light will be. If the lens focuses the light on the retina (because of the shape of the cornea with the length of the eyball, you will have 20/20 vision. The lens is not controlled by muscles, but by ligaments. As you age, the ability of the lens to focus more closely decreases (due to the ligaments becoming less flexible.) This is presbyopia (old vision), and the reason you will need reading glasses. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [October 13, 2009, 11:01pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/17 "2009-10-13T23:01:52Z")
Rods offer very little to improve acuity, and there are very few to none in the foveola (the very center of your vision). Cones provide a much larger input to acuity. The cornea provides about 2/3 of refractive power and the lens the remaining third. I’ve always heard that presbyopia is caused due to accomodation difficulties stemming from thickening and lack of elasticity in the lens itself, and not in the zonule. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [October 14, 2009, 9:44am UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/18 "2009-10-14T09:44:10Z")
OK. I knew that rods and cones allow for light and color, but as I remembered it is the cones that allow for color. My mnenomic is that “c” for color and cones, but perhaps I remembered incorrectly. I will quote from _Anatomy and Physiology_, Steen and Montagu, 1959: > [@](#): > > Mechanisms of Accommodation. The crystalline lens is held in a state of tension by the pull of the suspensory ligament. Fibers of this ligament extend radially from the margin of the lens to the ciliary processes, to which they are attached…In far vision, the light rays that enter the pupil of the eye are nearly parallel and are brought to a focus on the retina. In near vision, accommodation is necessary to bring about clear vision, which is accomplished by contraction of the ciliary muscle. The fibers of this muscle are so arranged that, when they contract, the ciliary body is pulled slightly forward, and this forward movement lessens the tension on the suspensory ligament, whereupon the lens, by virtue of its elasticity, becomes more spherical. This increases the convexity of the central portion of the anterior surface of the lens, increasing its refractive power and enabling light rays from objects to be brought to a focus on the retina… > > There are limits within which the eye can accommodate… For the average young adult, this averages about 10 inches; in infants it is much less (2 to 4 inches); in old people, it is much greater because the lens gradually loses its elasticity. So, you are correct. I’ve been mistaken. This book further states that myopia (nearsightedness) arises when the anteroposterior diameter of the eyeball is longer than usual, a condition which causes the light rays to come to a focus slightly in front of the retina. Hyperopia (farsightedness) arises when the vertical diameter of the eyeball is greater than usual, a condition which causes the light rays to come to a focus slightly behind the retina. Further reading in this book shows that my memory was correct: cones for color. > [@](#): > > When the image of a well-lighted object falls on the fovea or the part of the retina immediately surrounding it, all the spectral colors of the light emitted by the object can be distinguished. The cones in this area are especially adapted for responding to light of all wave lengths… Color discrimination is correlated to the distribution of the cones in the retina, the cones being most numerous in the fovea, absent in the peripheral regions… > > Visual acuity is greatest in the fovea, where only cones are present and where they are most concentrated. So, although the cones account for color, they also aid in visual acuity. My informant of years ago apparently said more cones and not more rods, and I did not remember correctly. Incidentally, referring to a prior post, the above shows that it is the focus point on the retina that is responsible for 20/20 vision, and that is not just an average. Corrective eye surgery flattens the cornea for myopics so that the focus point is further back, on the retina. The initial surgery was radial keratotomy (which is what I had), consisting of cutting slits in the cornea. It was discovered by a Russion (IIRC) accidentally that this would cause the cornea to flatten. New surgeries involve lasers which cut off layers from the cornea. Hyperopia was not amenable to correction by those methods, but I understand that this can now be corrected, too. Certainly, RK could not do that, but apparently they have a way with the laser surgeries (photorefractive radial keratotmy, lasik, etc.) to do this. ---
**Author:** ![cromulent](https://avatars.discourse-cdn.com/v4/letter/c/b4bc9f/32.png) [@cromulent](https://boards.straightdope.com/u/cromulent)\ **Post date:** [October 14, 2009, 1:23pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/19 "2009-10-14T13:23:17Z")
> [@barbitu8](#): > > This book further states that myopia (nearsightedness) arises when the anteroposterior diameter of the eyeball is longer than usual, a condition which causes the light rays to come to a focus slightly in front of the retina. Hyperopia (farsightedness) arises when the vertical diameter of the eyeball is greater than usual, a condition which causes the light rays to come to a focus slightly behind the retina. Your book is correct for the most part, but with an addendum – Most refractive error is due to axial length of the eye, but it’s also possible for the cornea to be either too steep or too flat, respectively. The majority of myopes have eyeballs that are too long, but a subset do have too-steep corneas (or a combination of the two.) The enlarged axial length is responsible for many of the retinal problems that high myopes are prone to, including retinal thinning, lattice degeneration, holes and detatchments, etc. ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [October 14, 2009, 7:31pm UTC](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336/20 "2009-10-14T19:31:18Z")
Since some with expertise in eyes are watching this thread, I will take the liberty of somewhat pirating it. A few years ago, my optometrist took a computerized picture of my eye and saw what he thought was drusen, very close to my macula. He was fearful I was developing macular degeneration. He referred me to a retina specialist who diagnosed myopic degeneration. He said that although RK had reduced my myopia to a management amount (just astigmatism in the left and -2 in the right), I still had the eye configuration for the greater myopia I once had, presumably he meant a long axial length. He said I could call it “drusen” if I like, but that implies macular degeneration. My question is whether the drusen is caused by macular degeneration or myopic degeneration, it is still drusen, and if it covers the macula, wouldn’t it cause the same symptoms? I haven’t worried about it since the opthalmologist wasn’t. [Next page](https://boards.straightdope.com/t/can-you-tell-your-20-20-eye-acuity-from-your-glasses-prescription/513336.md?page=2)