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Medical Forum / General / Vision / December 2006Can difference in vision be explained?
Is there a scientific explanation for why two people with the same prescription appear to have different levels of vision? For example, I've read somewhere that two people who are -3 won't necessarily see the same without glasses, one might see better/worse than the other. What factors influence this? > Is there a scientific explanation for why two people with the same > prescription appear to have different levels of vision? For example, I've > read somewhere that two people who are -3 won't necessarily see the same > without glasses, one might see better/worse than the other. What factors > influence this? OK, here is something for starters. There are optical limits and neurological limits. There are many variations in people's optical systems. Currently, the best way to measure this is with something called aberrometry, which is quite a bit more sophisticated than the methods used in the traditional eye exam. The pupil also plays an important role in fine-tuning the optical system. The pupil limits the influence that optical perfections have in distorting vision. Saying that two people have the same prescription is like saying that two people look the same because they have the same height or the same weight. It ignores the facial features. DrG Dear Katy, In fact there are two DIFFERENT methods of judging a person's refractive STATE. 1. Use a Snellen and a trial lens kit. (The so-called "subjective" measurement). In this method you have the person read the snellen at say 20/50. You then use stronger minus lenses until the 20/20 line is cleared, with say a -3/4 diopter lens.) 2. Measure with a retinoscope/cycloplegic (drops), and determine a person's refractive STATE. These two methods often do not agree -- and can be profoundly different. In fact it is possible to have 20/20 and a reafractive STATE (cycloplegic) of -3/4 diopters. This my explain some of the difference between your refractive measurement and your current visual acuity. It is also true as suggested by the ODs on sci.med.vision, that as you get older your refractive STATE will change in a positive direction, and your Snellen will clear. That might also explain why you can get along with no minus lens. Obviously, opinions vary concerning your distant visual acuity. Best, Otis > Is there a scientific explanation for why two people with the same > prescription appear to have different levels of vision? For example, I've [quoted text clipped - 5 lines] > Message posted via MedKB.com > http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 Dear Katy, I would also add, that sometimes the difference between Snellen and Retinoscope/cycloplegic can be profound. For instance, one 2 year-old had a Snellen of 20/60 -- which is functional for a child of that age -- and a retinoscope/cycloplegic of -11 diopters. So what do you do? Tell the mother that 20/60 is OK for now, or "prescribe" based on the retinoscope/cycloplegic. For the case in question, the child is now wearing a -10 dioper lens all the time. Obviously there is a majority and second opinion on this subject. Just one man's opinion. Best, Otis > Dear Katy, > [quoted text clipped - 44 lines] > > Message posted via MedKB.com > > http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 Statement of a "concerned" Parent of 3 year-old: I have posted several threads about my 3 year old on this site(-10.5 but sees 20/60 without glasses). My question is that a year ago when I brought my daughter in to the pediatric opthalmologist he originally had her perscription at -12.50 each eye. He gave her a lighter perscription to start out with though(-10.00). About 3 months later she was -11.50 each eye. About 3 months later he had her at about -11.00 each eye. Then about 3 months later he had her at -10.50 each eye. Just recently when she went back she was -10. 25 in one eye and -10.00 in the other. The astigmatism has been about the same for all perscriptions. I was not at the last visit and would have asked him why its going down. I am not posting this in ANYWAY to start any controversy. We have not been doing ANY sort of vision therapy or trying to improve her vision in anyway, shape, or form. I was wondering if it made any difference that she has lenticular myopia or if because she is young maybe her lens could be changing still. Signed, A concerned parent > Dear Katy, > [quoted text clipped - 68 lines] > > > Message posted via MedKB.com > > > http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 On 12/29/06 7:41 AM, in article 1167406898.484719.25500@i12g2000cwa.googlegroups.com, "otisbrown@pa.net" > For instance, one 2 year-old had a Snellen of > 20/60 -- which is functional for a child of that age -- and > a retinoscope/cycloplegic of -11 diopters. How can you even rely upon a 2 year-old to read a Snellen chart correctly? Bill -- Fermez le Bush > On 12/29/06 7:41 AM, in article > 1167406898.484719.25500@i12g2000cwa.googlegroups.com, "otisbrown@pa.net" [quoted text clipped - 4 lines] > > How can you even rely upon a 2 year-old to read a Snellen chart correctly? His mind is made up. Please do not try to confuse him with logic or facts. His head could explode.... > His mind is made up. Please do not try to confuse him with logic or > facts. His head could explode.... OK, I am finally willing to admit that the load of bricks might be missing a few.... DrG Good question. There are Snellens that have pictures. By these means an estimate of visual acuity can be obtained. I would bet that if this two-year old were the daughter of some of these ODs -- they would do a lot more checking before they put their child into a -10 diopter lens. In fact I do believe that some children have an adverse reaction to these drugs, that "freeze" the accommodation at the maximum "near" position of -10 diopters. But let us say that some second-opinion optometrists would suggest "holding off" on plunging a child that age into a -10 diopter lens. Second-opinions vary on this subject. Otis > On 12/29/06 7:41 AM, in article > 1167406898.484719.25500@i12g2000cwa.googlegroups.com, "otisbrown@pa.net" [quoted text clipped - 7 lines] > Bill > -- Fermez le Bush I have noticed that Otis always likes to make that last post at night, here in the U.S. He does this, no doubt, for all of his fans in Asia, who can then ponder his deep thoughts about the second opinion. DrG > Good question. > [quoted text clipped - 31 lines] > > Bill > > -- Fermez le Bush > I would bet that if this two-year old were the > daughter of some of these ODs -- they > would do a lot more checking before > they put their child into a -10 diopter lens. So this two-year-old was checked only once? And she's wearing them because an OD recommended it? > In fact I do believe that some children have > an adverse reaction to these drugs, that > "freeze" the accommodation at the > maximum "near" position of -10 diopters. Some children accommodate excessively after cyclopentolate? That would make a nice article, if you could find one. > But let us say that some second-opinion > optometrists would suggest "holding off" > on plunging a child that age > into a -10 diopter lens. And some would say we did. And some would say it isn't your business. -MT > I would also add, that sometimes the difference between > Snellen and Retinoscope/cycloplegic can be profound. NEARLY as profound as the difference between Otis's statements and actual fact. Nearly. > In fact there are two DIFFERENT methods of judging > a person's refractive STATE. Yes but your two "methods" don't really address her question, do they? She is asking why you can't predict VA from refraction. > then use stronger minus lenses until the 20/20 > line is cleared, with say a -3/4 diopter lens.) > 2. Measure with a retinoscope/cycloplegic (drops), > and determine a person's refractive STATE. Is a cycloplegic necessary to do retinoscopy? Can't I use lenses with cycloplegic? > These two methods often do not agree -- and > can be profoundly different. Perhaps because they don't measure the same thing. > In fact it is possible to have 20/20 and a reafractive > STATE (cycloplegic) of -3/4 diopters. And it's possible you don't know what you're talking about. > This my explain some of the difference between > your refractive measurement and your current > visual acuity. Not a whit. Pick either of your "methods," then find two people who measure the same. Why does one see better than the other without glasses? Hint: Answers include pupil size, higher-order aberrations, accommodative spasm and erroneous refraction, plus neurology, and psychology, and there's also the possibility that measured VA might NOT vary quite as much as she thinks it does. > Obviously, opinions vary concerning your > distant visual acuity. She wasn't asking for opinions. She was asking why subjective acuity varies from one person to another your refractive STATE is the same. -MT >> Is there a scientific explanation for why two people with the same >> prescription appear to have different levels of vision? For example, [quoted text clipped - 6 lines] >> Message posted via MedKB.com >> http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 > > In fact there are two DIFFERENT methods of judging > > a person's refractive STATE. > > Yes but your two "methods" don't really address her question, do they? > > She is asking why you can't predict VA from refraction. My wife was a chemist. Her boss was also. He wanted a formula to convert volume into weight. She couldn't do that. >>> In fact there are two DIFFERENT methods of judging >>> a person's refractive STATE. [quoted text clipped - 4 lines] > My wife was a chemist. Her boss was also. He wanted a formula to > convert volume into weight. She couldn't do that. Otis could. He's the poster child for density. >>> In fact there are two DIFFERENT methods of judging >>> a person's refractive STATE. [quoted text clipped - 5 lines] > My wife was a chemist. Her boss was also. He wanted a formula to > convert volume into weight. She couldn't do that. I bet she couldn't write him a formula to calculate the length of a piece of string, either...  SignatureRegards, Nicolaas. ... The real art of conversation is not only to say the right thing at the right time, but also to leave unsaid the wrong thing at a tempting moment. > Is there a scientific explanation for why two people with the same > prescription appear to have different levels of vision? For example, I've > read somewhere that two people who are -3 won't necessarily see the same > without glasses, one might see better/worse than the other. What factors > influence this? There are a lot of factors, for instance cataract. -3 doesn't tell how well someone sees, it is just the correction that gives best vision for that individual. Don't pay any attention to Otis. All other things being equal, pupil size is far and away the biggest determiner of unaided acuity between two myopes. Just like a camera, if you set the focus at a particular distance, changing the f-stop will profoundly affect the photo. That's why cheap cameras with tiny apertures don't really need any focusing mechanism, while expensive ones with large lenses and variable f-stops need such fine focusing. In the extreme example, take the -3 patient and have them look through a pinhole. They will be close to 20/20 without any lens at all. Take the pinhole away and they'd be lucky to get 20/200. w.stacy, o.d. >Is there a scientific explanation for why two people with the same >prescription appear to have different levels of vision? For example, I've [quoted text clipped - 3 lines] > > Oh, and this effect can be (and often is) utilized to distort acuity measurements. Allow the patient to squint (squeeze the lids together), and the reduction in the "aperture" will allow him to see smaller letters. This is why you see myopes squinting a lot when their glasses are too weak or when they don't have them. Similarly, flip on the overhead lighting and the pupils constrict, giving better acuity. Turn all lights off, and they do worse. The other big difference is psychological. Some people are far more profoundly bothered by blur than others. Some will feel completely happy in a -3 fog while others will feel like a fish out of water. w.stacy, o.d. > Take the pinhole away and they'd be lucky to get 20/200. William Stacy <wstacy@obase.net> wrote in news:b4hlh.41115$wc5.587 @newssvr25.news.prodigy.net: > This is why you see myopes squinting a lot when their glasses > are too weak or when they don't have them But doesn't the squinting cause the eyeball to shrink and cure the myopia?? Sorry Bill, long week, and no self control left SignatureScott Reverse name to reply > are too weak or when they don't have them. Similarly, flip on the > overhead lighting and the pupils constrict, giving better acuity. Turn > all lights off, and they do worse. I have noticed for a long time that my distance vision, which is corrected with glasses, is degraded when driving at night. Is the above the explanation for that? If so how can one compensate for the problem - different prescriptions for day and night use? > I have noticed for a long time that my distance vision, which is corrected > with glasses, is degraded when driving at night. Is the above the > explanation for that? If so how can one compensate for the problem - > different prescriptions for day and night use? You could, but a precise prescription should work for both. In your circumstances, it's often that the glasses are a little off but it's only evident when the pupils are large and depth-of-field is short. -MT |
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