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Medical Forum / General / Vision / March 2009Corrected for Distance or Near?
Hi, In prescribing glasses for myopes. The prescription says corrected for distance and not near. The reason being that when corrected for distance, there is less convergence needed when viewing near. Isn't there any side effect for getting less convergence compared to natural. Is the relaxation that one gets for reduce convergence significant? It really gives you less strain? I'm 29 years old and use computer 15 hours a day. I plan to get a duplicate pair of glasses with same -7.0 diopters but corrected for near to be free of glasses distortion but I won't be able to get the advantage of the glasses corrected for distance which is reduced convergence. What would you rather have, less distortion (corrected at near) or reduced convergence (corrected at distance) assuming you are a young myope? Hope someone can clear this up. Thanks a lot, Horus > In prescribing glasses for myopes. The prescription says corrected > for distance and not near. I don't understand. Is there some checkbox, or other indication that your glasses are not corrected for near? > The reason being that when corrected > for distance, there is less convergence needed when viewing > near. I didn't know that. My prescriptions don't specify convergence. > Isn't there any side effect for getting less convergence > compared to natural. -7.00 glasses assist near convergence automatically. When the glasses are made for a 64 pupil distance and you converge, your actual pupil distance becomes 60 or 61 and there is a tiny amount of base-in prism induced by decentration. > Is the relaxation that one gets for reduce > convergence significant? It really gives you less strain? Not normally. If you have an insufficiency of convergence ability or some other binocular disorder, it could be "less straining." In normal, healthy eyes it would be difficult to tell any difference. > I'm > 29 years old and use computer 15 hours a day. I plan to get > a duplicate pair of glasses with same -7.0 diopters but corrected > for near to be free of glasses distortion I don't think you'll gain much from changing the pupil distance, if both pairs are -7.00. In most cases, "reducing strain" indicates reducing accommodation. Accommodating stimulates convergence and it will still continue to do so if you make new glasses. So you risk tipping over into convergence excess, rather than convergence insufficiency. > but I won't be able to get > the advantage of the glasses corrected for distance which is > reduced convergence. Reducing convergence will have no net effect on "distortion" in -7.00 glasses. > What would you rather have, less > distortion (corrected at near) or reduced convergence > (corrected at distance) assuming you are a young myope? > Hope someone can clear this up. The problem is you're overthinking. -MT > > In prescribing glasses for myopes. The prescription says corrected > > for distance and not near. > > I don't understand. Is there some checkbox, or other indication that your > glasses are not corrected for near? What I meant to say is this. My PD is 68. The glasses optical centers are also 68 (meaning corrected for distance although one can still focus near of course). Now corrected for near means the optical centers are 68-2=66 or 65. For Myopes with -7.0 diopter lens right and -8.0 diopter lens left that uses computer a lot. Does it make sense to get a 65 (corrected for near) pair rather than corrected for far (68) if my PD is 68? > > The reason being that when corrected > > for distance, there is less convergence needed when viewing [quoted text clipped - 33 lines] > So you risk tipping over into convergence excess, rather than convergence > insufficiency. I don't understand. My PD is 68. If I get a pair with optical centers of 66. This means my eyes are looking thru the optical centers when seeing near. Convergence will be normal. But if I use a pair with optical centers same as my PD. There will be less convergence demand due to the base-in as you stated. Isn't this the important reason why glasses are corrected for distance (meaning the PD matches the optical centers) in order to make you more relax even if you will use the minus lens exclusively for near?? > > but I won't be able to get > > the advantage of the glasses corrected for distance which is > > reduced convergence. > > Reducing convergence will have no net effect on "distortion" in -7.00 > glasses. What I meant by distortion is not seeing directly at the optical centers but sideways (or base in). > > What would you rather have, less > > distortion (corrected at near) or reduced convergence [quoted text clipped - 4 lines] > > -MT Thanks, -Horus > right and -8.0 diopter lens left that uses computer a lot. Does it > make sense to get a 65 (corrected for near) pair rather than > corrected for far (68) if my PD is 68? I'm pretty sure you couldn't tell the difference. Your glasses (pd 68) provide about 2.5 prism diopters of "assistance" when you converge to a PD of 65. Remaking them with a PD of 65 would eliminate that assistance and require you to converge _more._ > I don't understand. My PD is 68. If I get a pair with optical centers > of [quoted text clipped - 5 lines] > matches the optical centers) in order to make you more relax > even if you will use the minus lens exclusively for near?? I don't think it's done for that reason. If you're farsighted, the same conventions work _against_ you. > What I meant by distortion is not seeing directly at the optical > centers but sideways (or base in). Regardless of centration and prism, there is huge barrel distortion at -700 or -800. Moving the OCs only moves the center of the "barrel" a little left or right. Doesn't reduce it at all. -MT > > right and -8.0 diopter lens left that uses computer a lot. Does it > > make sense to get a 65 (corrected for near) pair rather than > > corrected for far (68) if my PD is 68? > > I'm pretty sure you couldn't tell the difference. Couldn't tell the difference of what? Couldn't tell or feel the difference of the covergence assistance? > Your glasses (pd 68) provide about 2.5 prism diopters of "assistance" when > you converge to a PD of 65. > > Remaking them with a PD of 65 would eliminate that assistance and require > you to converge _more._ But you said above I couldn't tell the difference. Or do you mean other thing beside convergence assitance, what is it that I couldn't tell the difference? > > I don't understand. My PD is 68. If I get a pair with optical centers > > of [quoted text clipped - 15 lines] > or -800. Moving the OCs only moves the center of the "barrel" a little left > or right. Doesn't reduce it at all. My myopic power is -7.0D on right and -8.0D on left. Does the inbalance power mean the convergence on each eye is different? In such cases, what are usually done to the eyeglasses optical centers? If my PD is 68. Should the glasses oc still be 68 or should it be adjusted to compensate for mismatched power of -7.0D and -8.0D? You mentioned "Your glasses (pd 68) provide about 2.5 prism diopters of "assistance" when you converge to a PD of 65". How did you solve for the value of 2.5 prism diopters of "assistance". How large should prism diopters be before one can significantly feel the assistance? If one's PD is 68 and one wants an extra pair for near view to take advantage of the assistance. Does it make sense to make the optical centers of the extra pair even wider than 68 to get more base-in assistance or reduced convergence demand? How wider (72? 75?) for optimum prism diopters/assistance effect? Many thanks, Horus > -MT >> I'm pretty sure you couldn't tell the difference. > Couldn't tell the difference of what? Couldn't tell or feel the > difference of the covergence assistance? Right. With glasses of ~-7.50, I doubt you could feel a difference between glasses made with PD of 68 and a pair made with 65. At least when reading, probably no difference. It's just as likely they'd be _uncomfortable_ for distance vision. > Remaking them with a PD of 65 would eliminate that assistance and require > you to converge _more._ > But you said above I couldn't tell the difference. Or do you > mean other > thing beside convergence assitance, what is it that I couldn't > tell the difference? I mean that theoretically you will have to converge MORE with glasses made at 65 mm. You would also have to converge MORE if you glance up and watch TV far away. > My myopic power is -7.0D on right and -8.0D on left. Does the > inbalance power mean the convergence on each eye is different? For ease of calculation (induced prism, Prentiss' rule) you could assume both lenses are -7.50. > In such cases, what are usually done to the eyeglasses optical > centers? If my PD is 68. Should the glasses oc still be 68 or > should it be adjusted to compensate for mismatched power > of -7.0D and -8.0D? The PD should be 68. > You mentioned "Your glasses (pd 68) provide about 2.5 prism diopters > of "assistance" when you converge to a PD of 65". > How did you solve for the value of 2.5 prism diopters of "assistance". > How large should prism diopters be before one can significantly feel > the assistance? Prentiss' rule says diopters x decentration in cm = prism. 7.50 x 0.3 = 2.25 prism diopters. You could get fancy and figure each eye separately but you'll still round prism diopters to whole units, unless you want to give the lab guys a laugh. > If one's PD is 68 and one wants an extra pair for near view to > take advantage of the assistance. Does it make sense to make > the optical centers of the extra pair even wider than 68 to get > more base-in assistance or reduced convergence demand? Unless you have a complaint like headache or diplopia, you're obsessing. Convergence is normally inseparable from accommodation. If you accommodate, you converge. If you converge, you accommodate. If you alter one side of that relationship without considering the other it's like running a bicycle with wheels of different sizes. Prism is prism, whether it's obtained by decentration or by actually altering the grind. Let's put Prentiss' rule aside. You seem to be saying that your vision would be better if you wore a few p.d. of horizontal prism. Why do you need to alter your vergence demand at near? We'll want to know your phoria. What is your phoria at near? You can test it by looking at one single spot on a page, then watch the spot while you alternately cover each eye. Don't allow yourself to use both eyes during the test. Switch your occluder quickly, from one eye to the other, and measure the distance your two images "jump." That "jump" is your phoria, a resting position that depends on your anatomy and on your binocular reflexes. These reflexes have _learned_ to coordinate your accommodation and your convergence simultaneously, and somewhat inseparably. You can predict that people with _big_ phorias (jumps) will appreciate prism. To get a better prediction, you can test phoria at distance and near. But even with good, predictive formulas we don't go fixing stuff that ain't broke. Have any discomfort, double vision? What are you trying to fix? -MT > >> I'm pretty sure you couldn't tell the difference. > > Couldn't tell the difference of what? Couldn't tell or feel the [quoted text clipped - 17 lines] > > You would also have to converge MORE if you glance up and watch TV far away. My PD is 68 and if I get extra pair made at 65mm. It will be exclusively for near use and not far because I have the other glass made at 68. I thought that there would be less chromatic aberration if I get one made at 65. So you are saying it's not much and impractical to make one at 65 and you never made one for any of your patient because there is absolutely no advantage of any kind? > > My myopic power is -7.0D on right and -8.0D on left. Does the > > inbalance power mean the convergence on each eye is different? [quoted text clipped - 21 lines] > You could get fancy and figure each eye separately but you'll still round > prism diopters to whole units, unless you want to give the lab guys a laugh. Know the relationship of convergence and power. Supposed a patient had a myopia of only -1.5D at right side and -8.0D at left side. Can the convergence change? In this case, how is the optical centers of the lens adjusted (if at all)? Just want a good theorerical grasp of relationship between convergence and power. > > If one's PD is 68 and one wants an extra pair for near view to > > take advantage of the assistance. Does it make sense to make > > the optical centers of the extra pair even wider than 68 to get > > more base-in assistance or reduced convergence demand? > > Unless you have a complaint like headache or diplopia, you're obsessing. I use computer 15 hours a day. Convergence may cause eyestrain so I thought if I can make the glasses wider and induce more base-in for even more reduced convergence demand, there would be less eyestrain. Isn't this done in practice to get less eyestrain? > Convergence is normally inseparable from accommodation. If you accommodate, > you converge. If you converge, you accommodate. If you alter one side of > that relationship without considering the other it's like running a bicycle > with wheels of different sizes. You mentioned yesterday: "I don't think you'll gain much from changing the pupil distance, if both pairs are -7.00. In most cases, "reducing strain" indicates reducing accommodation. Accommodating stimulates convergence and it will still continue to do so if you make new glasses. So you risk tipping over into convergence excess, rather than convergence insufficiency. " Why did you mention "convergence excess"? I mean, if I make an extra pair of 65 oc (my pd is 68). The convergence would be similar to people who don't use glasses, right? Or are you saying that using glasses when reading where the oc is 65 (looking directly at optical centers) would produce more convergence than without using glasses (supposed there were no myopia)? > Prism is prism, whether it's obtained by decentration or by actually > altering the grind. Let's put Prentiss' rule aside. > > You seem to be saying that your vision would be better if you wore a few > p.d. of horizontal prism. Why do you need to alter your vergence demand at > near? As I have said, just to get less eyestrain by reducing convergence and accomodation when using monitor 15 hours a day, nothing more. I have no abnormal phoria. Thanks, Horus > We'll want to know your phoria. What is your phoria at near? > [quoted text clipped - 18 lines] > > -MT In article <c72c3580-c936-4f32-a582-ad3202f6a345@s38g2000prg.googlegroups.com>, > My PD is 68 and if I get extra pair made at 65mm. It will be > exclusively [quoted text clipped - 6 lines] > > > My myopic power is -7.0D on right and -8.0D on left. Does the > > > inbalance power mean the convergence on each eye is different? > Know the relationship of convergence and power. Supposed > a patient had a myopia of only -1.5D at right side and > -8.0D at left side. Can the convergence change? In > this case, how is the optical centers of the lens > adjusted (if at all)? Just want a good theorerical > grasp of relationship between convergence and power. I've tried that. I was zero in the right, and -10D in the left. Wearing glasses, it is impossible for the brain to merge the two images, due to the difference in size of the images. It doesn't matter what you do, it cannot be done. I went through five years of seeing double, until my brain learned how to just block out one image completely. I didn't have this problem with contacts, and so wore them 14 hours a day, 7 days a week. This was before the 30 day contacts came out. > In most cases, "reducing strain" indicates reducing accommodation. > Accommodating stimulates convergence and it will still continue to do > so if > you make new glasses. If you want to reduce accommodation, why not just reduce accommodation? Currently you are wearing -7D and -8D to correct your vision at infinity. Find out what you need to see at your normal close distance. Perhaps it might be -5D and -6D. > > You seem to be saying that your vision would be better if you wore a few > > p.d. of horizontal prism. Why do you need to alter your vergence demand at [quoted text clipped - 4 lines] > 15 hours a day, nothing more. I have no abnormal > phoria. > > What are you trying to fix?  SignatureDan Abel Petaluma, California USA dabel@sonic.net > In article > <c72c3580-c936-4f32-a582-ad3202f6a...@s38g2000prg.googlegroups.com>, [quoted text clipped - 17 lines] > > I've tried that. I was zero in the right, and -10D in the left. First time for me to hear a person who has -10D myopia in the left and 0 in the right. You are kidding right? If you are not kidding. Your unique situation may give researchers clue to whether a single genetic switch can initiate eyeball shape change and learn the trigger. Perhaps you always sleep on the left side or use appliance on the left that could trigger the genetic change that make your left eye abnormal? What other theories do you have why you have perfect right and incredibly high myopic left? Hope you can make meet some eye researchers for your unique situation. Horus > Wearing glasses, it is impossible for the brain to merge the two images, > due to the difference in size of the images. It doesn't matter what you [quoted text clipped - 29 lines] > > - Show quoted text - > > In article > > <c72c3580-c936-4f32-a582-ad3202f6a...@s38g2000prg.googlegroups.com>, [quoted text clipped - 67 lines] > > > - Show quoted text - For what it's worth... Horus's IP Address puts him/her in the Philipines. I wouldn't rule out this BEING Pia, again, but under an alias. Would that surprise anybody, btw? Nah.... > Would that surprise anybody, btw? > Nah.... Who are you? :) -MT > > Would that surprise anybody, btw? > > Nah.... > > Who are you? :) > > -MT Imelda's long-lost stepson. You know: the guy who -- in an effort to break from the stain of my family's tradition -- occasionally comes to s.m.v., and declares that shoes ARE the cause of all myopia. > You know: the guy who -- in an effort to break from the stain of my > family's tradition -- occasionally comes to s.m.v., and declares that > shoes ARE the cause of all myopia. Sure, but that's _this_ week. So you won't be surprised to learn that LensCrafters' first parent corp was U.S. Shoe? -MT >>> Would that surprise anybody, btw? >>> Nah.... [quoted text clipped - 8 lines] > family's tradition -- occasionally comes to s.m.v., and declares that > shoes ARE the cause of all myopia. Aw, hell ... and all along we thought it was caused by indigestion and tight underpants! Signature- Nicolaas In article <1c1a8c84-5d2e-4e53-92cd-df71187c4cb4@n33g2000pri.googlegroups.com>, > > > I've tried that. I was zero in the right, and -10D in the left. > > > > First time for me to hear a person who has -10D myopia > > in the left and 0 in the right. You are kidding right? It's not common, and I'm not kidding. Furthermore, it was my choice. > > If you are not kidding. Your unique situation may give > > researchers clue to whether a single genetic switch [quoted text clipped - 6 lines] > > can make meet some eye researchers for your > > unique situation. Wow! A whole life story based on one short sentence! You are very creative, indeed. I had surgery in the right eye, and asked the doctor to give me distance vision (zero myopia). He was not willing to do surgery on the left eye until the cataract had progressed further. As I saw well with contacts (which the doctor had the foresight to strongly suggest to me, years before the first surgery), it wasn't critical. Five years later he did surgery in the left eye, and they now match in myopia (zero). > > > Wearing glasses, it is impossible for the brain to merge the two images, > > > due to the difference in size of the images. It doesn't matter what you [quoted text clipped - 12 lines] > > > infinity. Find out what you need to see at your normal close distance. > > > Perhaps it might be -5D and -6D. > For what it's worth... > [quoted text clipped - 5 lines] > > Nah.... I looked at their headers, and they are quite similar. Of course, a lot of people use GoogleGroups, and their headers should look similar. They both use exactly the same software (including version number and extensions). Again, a lot of people use MS Windows with the same version. SignatureDan Abel Petaluma, California USA dabel@sonic.net In article <dabel-A7D7ED.12085820032009@c-61-68-245-199.per.connect.net.au>, Dan Abel <dabel@sonic.net> wrote: > I looked at their headers, and they are quite similar. Of course, a lot > of people use GoogleGroups, and their headers should look similar. They > both use exactly the same software (including version number and > extensions). Again, a lot of people use MS Windows with the same > version. It is the same person. Pia is Horus. SignatureCharles > In article > <dabel-A7D7ED.12085820032009@c-61-68-245-199.per.connect.net.au>, Dan [quoted text clipped - 7 lines] > > It is the same person. Pia is Horus. OK, you've got me curious. Other than the fact that they seem to be the same person, is there some clue, or set of clues, that you used to determine that they *were* in fact the same person? SignatureDan Abel Petaluma, California USA dabel@sonic.net In article <dabel-C4D6D8.17484420032009@c-61-68-245-199.per.connect.net.au>, Dan Abel <dabel@sonic.net> wrote: > OK, you've got me curious. Other than the fact that they seem to be the > same person, is there some clue, or set of clues, that you used to > determine that they *were* in fact the same person? In addition to that they seem to be the same person, their IP address is from the same block and traces back to the same internet provider in the Philippines, pldt.net. Right there would be enough for me but both using the same software according their headers including Media Center PC 5.0 version of Vista. There can't be that many coincidences. SignatureCharles > In article > <dabel-C4D6D8.17484420032...@c-61-68-245-199.per.connect.net.au>, Dan [quoted text clipped - 12 lines] > -- > Charles With all that, and they both wear RGP lenses to boot? Ding! Ding! Ding! Ding! I think we have us a winner!! :) Should we change the name to Horusia, or Porus?? -=# Firewalker #=- > I thought that there would be less chromatic aberration if I get > one made at 65. I don't see why. If you have color fringes around objects, that's chromatic aberration. If that's the problem you're likely wearing polycarbonate and your only solution is to get rid of the polycarbonate. Chromatic abb doesn't usually cause eyestrain. > So you are saying it's not much and impractical > to make one at 65 and you never made one for any of your > patient because there is absolutely no advantage of any kind? Probably not. Like I said, you probably wouldn't feel any difference. When eyestrain is a problem, we usually approach it by reducing _accommodation_, not convergence. > Know the relationship of convergence and power. Supposed > a patient had a myopia of only -1.5D at right side and > -8.0D at left side. Can the convergence change? A prism diopter or two. Prescriptions like that have other problems, and altering convergence doesn't fix it. > I use computer 15 hours a day. Convergence may cause > eyestrain so I thought if I can make the glasses wider > and induce more base-in for even more reduced > convergence demand, there would be less eyestrain. > Isn't this done in practice to get less eyestrain? Notice how you delicately skirt around the question - you only say that "convergence may cause eyestrain," not that you actually have it. In practice we usually adjust the spectacle power to help eyestrain. PD adjustments are a relative afterthought. > Convergence is normally inseparable from accommodation. If you > accommodate, > you converge. If you converge, you accommodate. If you alter one side of > that relationship without considering the other it's like running a > bicycle > with wheels of different sizes. > Why did you mention "convergence excess"? I mean, if I > make an extra pair of 65 oc (my pd is 68). The convergence [quoted text clipped - 3 lines] > would produce more convergence than without using > glasses (supposed there were no myopia)? No, but it WOULD require more convergence if you happened to look across the room at TV, which you would inevitably do. > As I have said, just to get less eyestrain by reducing > convergence and accomodation when using monitor > 15 hours a day, nothing more. I have no abnormal > phoria. Typically we reduce "eyestrain" by reducing accommodation. Changing the phoria with induced prism is only likely to help if there is a phoria problem. Likewise changing the accommodation by changing the power is only likely to help if there's an accommodation problem. If we change one, we usually change the other because accommodation and convergence are wired together. Even so, accommodating and converging all day doesn't guarantee eyestrain. You haven't actually said you have eyestrain. -MT > > I thought that there would be less chromatic aberration if I get > > one made at 65. [quoted text clipped - 16 lines] > When eyestrain is a problem, we usually approach it by reducing > _accommodation_, not convergence. But you said accomodation and convergence are wired together and changing one can change the other. So if you reduce convergence, you reduce accomodation. But you said that in eyestrain, one must reduce accomodation. However we can reduce convergence and hence reduce accomodation. Unless you mean there is a way to reduce accomodation without reducing convergence? How? > > Know the relationship of convergence and power. Supposed > > a patient had a myopia of only -1.5D at right side and [quoted text clipped - 11 lines] > Notice how you delicately skirt around the question - you only say that > "convergence may cause eyestrain," not that you actually have it. Of course I have eyestrain daily when using computer 15 hours. I just want to know ways to eliminate it by altering the glasses. It doesn't make sense to induce more base-in to converge less by making the optical centers wider than the PD? I still can't see the reason why it can't be done. It's not commonly done in the 1980s but with computers now becoming part of our life and we using it 15 hours daily. We ought to think of it. Well. I just got back from an optical shop to get an extra pair to keep at house and car in case of emergency (I get lots of glasses for use at different times & situations). They can't get the optical centers the same vertical distance even when using automatic eyeglass making machine. It's off by 0.5mm to 1mm. Is this allowance normal in automatic eyeglass making machine or are certain machines more accurate than others? In your machine, can you produce 100% fully vertically centered OC, that is, without any 0.5mm-1mm off in the vertical axis of the optical centers? Thanks. Horus > In practice we usually adjust the spectacle power to help eyestrain. PD > adjustments are a relative afterthought. [quoted text clipped - 35 lines] > > -MT > But you said accomodation and convergence are wired together > and changing one can change the other. Clear vision is an equilibrium, a balance between convergence and accommodation. Equilibrium is a balance that can tip either way. > So if you reduce > convergence, you reduce accomodation. If you "assist" convergence, some people lose focus. > But you said > that in eyestrain, one must reduce accomodation. No, I said most "eyestrain" is accommodative, not phoria-related. IOW focusing (accommodative) problems are more common than convergence (diplopia) problems. Convergence is seldom treated with prism (by decentration or otherwise) when there is a normal phoria and no diplopia. > However > we can reduce convergence and hence reduce accomodation. That's right. Thing is, if you hold a book at 40 cm and _reduce_ your accommodation, THE PAGE GETS BLURRY. > Unless you mean there is a way to reduce accomodation > without reducing convergence? How? Add +0.50 to your prescription. Most of us wouldn't adjust the PD when doing that. > Of course I have eyestrain daily when using computer 15 hours. > I just want to know ways to eliminate it by altering the glasses. Have your phoria tested. If it's normal, have glasses adjusted by +0.50 and quit worrying about prism. > It doesn't make sense to induce more base-in to converge less > by making the optical centers wider than the PD? I still > can't see the reason why it can't be done. Because near work gets blurry when you reduce accommodation. If you reduce the need for convergence, you usually must reduce the need for accommodation by incorporating plus lenses. OTOH converging assists accommodation, so if someone doesn't accommodate well, we might INCREASE the demand for convergence, to boost accommodation. > It's not commonly > done in the 1980s but with computers now becoming > part of our life and we using it 15 hours daily. We ought > to think of it. Not as you imagine. Until computers, 20 years ago the "nominal reading distance" for near vision was 16 inches. For grandma reading her Bible, that's about right. Now Gramma has a PC and a Wii and wants PC glasses that are clear at 24". > It's off by 0.5mm to 1mm. Is this allowance normal > in automatic eyeglass making machine or are certain > machines more accurate than others? In your machine, > can you produce 100% fully vertically centered OC, > that is, without any 0.5mm-1mm off in the vertical > axis of the optical centers? Maybe things are different there, but I thought people made those placements, not machines. Anyhow... Test your phoria. If you don't have vertical phoria at the reading position in your glasses, the placement is fine. If you do have significant vertical phoria at your reading position, it'll get tiresome pretty quick and you'll want to ask for a remake. Wear them in, show the optician "things jump up and down" when you alternate your hand over one lens then the other. If the OCs are _perfect_ you can still always show vertical phoria in certain positions because your lenses aren't the same left and right. Don't evaluate at the OCs. Evaluate your phoria at the position you read through. -MT > > It's not commonly > > done in the 1980s but with computers now becoming [quoted text clipped - 5 lines] > Until computers, 20 years ago the "nominal reading distance" for near vision > was 16 inches. For grandma reading her Bible, that's about right. And when Grandma wasn't reading her bible, she was darning socks or doing other close work, like child care and food prep. It was darn seldom that she took her high powered rifle out hunting moose at 300 yards. > Now Gramma has a PC and a Wii and wants PC glasses that are clear at 24". Not to mention those distance glasses to see the aerobics instructor in her class. > > It's off by 0.5mm to 1mm. Take Mike's advice. Get contacts. All of your imaginary problems will disappear! And I strongly suspect that once you choose some nice stylish frames, you will find that .5mm is nothing compared to the differences in distance involved in the frame and your eyes. SignatureDan Abel Petaluma, California USA dabel@sonic.net > > But you said accomodation and convergence are wired together > > and changing one can change the other. [quoted text clipped - 79 lines] > > -MT Ok. Thanks for the above. I'll think of it. Here's a new experience I just had. After getting the extra pair (I'm myope), I was trying it out and I noticed for the first time something that is present in all eyeglasses, the images in the eyeglasses are smaller than the actual. I noticed this when I look at the monitor without glasses and as I slowly put the glasses from above, I can see the monitor as smaller (as seen thru glasses versus directly seeing it below the frame). Now questions: 1. Is the reason the monitor look bigger without using glasses is because the image is defocused and hence bigger? 2. If not, and reason being that the glasses are farther away, but if I put the glasses by pressing it to my eyes, I can still see the monitor as smaller. Why didn't it change bigger? 3. Can wearing contact lens give the same size perspective as actual for one to one size match? 4. How do you optimize your glasses to make the image you see as big as or close to the original.. such that seeing the monitor without glasses and putting the glasses slowly would make you see images size that are not far? 5. Does it have something to do with the power, meaning myopia with larger power (above -4.0D) would produce smaller image scale (or smaller monitor for example)? Thanks, Horus > noticed for the first time something that is present in all > eyeglasses, > the images in the eyeglasses are smaller than the actual. You betcha. > 1. Is the reason the monitor look bigger without using > glasses is because the image is defocused and hence > bigger? No, it's a different reason. > If not, and reason being that the glasses are farther > away, but if I put the glasses by pressing it to my eyes, > I can still see the monitor as smaller. Why didn't it change > bigger? Because you've adapted and become less sensitive to it. The formula says minification increases with distance. We're talking about spectacle magnification. There are lots of good tutorials on spec mag just a google-click away. The spec mag formula can be found at http://www.drdrbill.com/downloads/optics/ophth-optics/Spectacle_Magnification.pdf and elsewhere. Minus lenses minify. Plus lenses magnify. Hand-held magnifiers are plus lenses that make use of this property. But they don't magnify (much) until you lift them up off the paper. The amount of magnification (or minification) can be calculated from the power, thickness, refractive index, and distance to the lens. Adjust any one of those and you get major changes in mag. > 3. Can wearing contact lens give the same size > perspective as actual for one to one size match? Pretty much. The perfect location to add a corrective lens would be at the nodal point inside the eye, about what's done basically cataract surgery. Contacts are just about as good, without the surgery. > 4. How do you optimize your glasses to make the image > you see as big as or close to the original.. such that > seeing the monitor without glasses and putting the glasses > slowly would make you see images size that are not far? Make your glasses small, round, and closely fit, like they did in the 1880s. > 5. Does it have something to do with the power, meaning > myopia with larger power (above -4.0D) would produce > smaller image scale (or smaller monitor for example)? Spec mag is multiplied by power, so yes, of course. But we don't usually change the power just to change the mag. Mag problems show up as glasses that "make the floor too close" or "make the walls curved" or "make me feel I'm walking downhill". Most of those sensations go away completely after a few days or a week. The mag doesn't change over time, but your brain usually relearns its frame of reference. If not, you should look for a new frame that is smaller, rounder, and more closely fit. -MT > > But you said accomodation and convergence are wired together > > and changing one can change the other. [quoted text clipped - 73 lines] > want to ask for a remake. Wear them in, show the optician "things jump up > and down" when you alternate your hand over one lens then the other. When testing what you mentioned. Things don't really jump up and down, however they jump left and right when I alternate my hand over one lens then the other. Is this supposed to be normal experienced by all or only a few? The view focused is very far away (we can consider it as infinity or straight ahead). This means the horizontal OC is not aligned or is it the tilt? All my glasses show this. In a perfect spectacle.. if a patient view at infinity, isn't there any left and right jumping at all? My myopia is -7.0 and -8.0. Could the mismatch be the cause of the inbalance? No way to correct it? Anyway. Where in the United States can I order online where I simply give them the prescription... where they make perfect spectacles with zero horizontal jumping and the PD exactly dead on? Laboratory so accurate that the glasses are almost MilSpec. Any such place? Horus > If the OCs are _perfect_ you can still always show vertical phoria in > certain positions because your lenses aren't the same left and right. Don't > evaluate at the OCs. Evaluate your phoria at the position you read through. > > -MT > When testing what you mentioned. Things don't really jump up and > down, > however they jump left and right when I alternate my hand over one > lens > then the other. Is this supposed to be normal experienced by all > or only a few? That's your phoria. It's the same with every pair because it's the way your eyes line up, not because your glasses are wrong. A little horizontal jump is normal. That's what you meant when you said you have no abnormal phoria. > Anyway. Where in the United States can I order online where > I simply give them the prescription... where they make perfect > spectacles with zero horizontal jumping and the PD exactly > dead on? Laboratory so accurate that the glasses are almost > MilSpec. Any such place? You want extreme technical accuracy from an online retailer. I want a new boat. I doubt either of us will be satisfied very soon. -MT > > When testing what you mentioned. Things don't really jump up and > > down, [quoted text clipped - 6 lines] > eyes line up, not because your glasses are wrong. A little horizontal jump > is normal. That's what you meant when you said you have no abnormal phoria. I wonder how one can differentiate between glasses that are wrong and the eyes having phoria (?) Any particular tests? > > Anyway. Where in the United States can I order online where > > I simply give them the prescription... where they make perfect [quoted text clipped - 4 lines] > You want extreme technical accuracy from an online retailer. I want a new > boat. Well. I live in Indonesia. All optical shops can't promise accuracy so the vertical displacement is on average around 1-2mm. Since my power is high at -8.0. I can't tolerate even 1mm. I want zero vertical displacement which no one in the country can deliver. However you hint that in the US. Eyeglasses are accurately made so I wonder where I can order one in the US. Since I haven't set foot in your country. I can't just walk in to one. Hence I need to order it online. Maybe you can even make one for me with your zero displacement error technical knowhow and state of the art fabrication equipment? Horus > I doubt either of us will be satisfied very soon. > > -MT > I wonder how one can differentiate between glasses that are wrong > and the eyes having phoria (?) Any particular tests? If the question is burning you, you could measure phoria twice, once with glasses and once with contacts. > Well. I live in Indonesia. All optical shops can't promise accuracy so > the vertical displacement is on average around 1-2mm. Since my > power is high at -8.0. I can't tolerate even 1mm. I want zero vertical > displacement which no one in the country can deliver. When did you tell us you had vertical phoria while reading? If you have no vertical phoria or diplopia when reading, the measurements. do. not. matter. It sounds to me like you've had several pair that were made just fine and now you're looking to solve a problem you don't have. The US may have more regulation, but the standard for making eyeglasses is an ANSI document, adopted internationally. Micrometer accuracy is seldom necessary. What IS necessary is the experience and skill to make a comfortable pair of glasses. If you understand Prentiss' rule, you might understand how lenses of different powers (eg -700 and -800) ALWAYS create vertical phoria in up or down gaze. Theoretically the vertical phoria should be zero at the optical centers of the glasses. Once you understand that, check and see if you read through the optical centers of your glasses. No? You're reading through the lens at a LOWER position? Then you are CREATING induced prism and you've had vertical imbalance all along. You can tolerate a couple of prism diopters of vertical displacement without problem. > However > you hint that in the US. Eyeglasses are accurately made so I > wonder where I can order one in the US. Since I haven't > set foot in your country. I can't just walk in to one. Hence US dollars aren't very strong right now and you might find you could make 5 or 10 pairs of glasses there for the cost of one pair here. Look for an experienced optician and ask for a comfortable pair of glasses. -MT > > I wonder how one can differentiate between glasses that are wrong > > and the eyes having phoria (?) Any particular tests? > > If the question is burning you, you could measure phoria twice, once with > glasses and once with contacts. Yes done it with RGP contacts. Phoria with contact lens is almost nonexistent so the phoria with the glasses is caused by decentering. The reason I still need glasses even if I have contacts is because I can't use contacts 15 hours a day because of dry eyes. > > Well. I live in Indonesia. All optical shops can't promise accuracy so > > the vertical displacement is on average around 1-2mm. Since my [quoted text clipped - 3 lines] > When did you tell us you had vertical phoria while reading? If you have no > vertical phoria or diplopia when reading, the measurements. do. not. matter. By putting covering each eye alternately with one hand, I can see about 2mm vertical jump when using the glasses. No vertical jump in contact lens or just 0.05mm. > It sounds to me like you've had several pair that were made just fine and > now you're looking to solve a problem you don't have. Well. I have 4 pairs of glasses. One has horizontal displacement of 5mm, one by 3mm, one by 2mm, one also by 2mm. Vertical is off by 1mm. Now I want glasses that is dead center vertically and horizontally. Its ok if I have very slight phoria but at least I want my glasses to be accurate so the phoria won't be from many sources. Say. Have you encountered eyeglasses frames that have collimination screws that hold the lens? This means the lens are not stuck to the frame body but by the collimination screws entirely holding them (floating the lens just like in SCTs telescope)? This would enable manual adjustment with help of a lensometer for complete centering. I'm looking for this kind of frame that may be available someplace in the planet. Then I'll buy it and then have local optician get a lens for it. This is the ultimate in frame design and centering. > The US may have more regulation, but the standard for making eyeglasses is > an ANSI document, adopted internationally. Micrometer accuracy is seldom [quoted text clipped - 6 lines] > different powers (eg -700 and -800) ALWAYS create vertical phoria in up or > down gaze. Prentiss rule = power x vertical displacement If displacement is 1.5mm.. then prentiss for the left for example is -7.0 x 0.15 = 1.05 prism diopter. However if the -7 and -8 is dead center at the optical center. And vertical displacement is zero then prestiss rule = power x zero = zero. In this case, how do you solve for the vertical prism diopter of the -7 and -8 when there is no displacement in distance but by power?? > Theoretically the vertical phoria should be zero at the optical centers of > the glasses. [quoted text clipped - 11 lines] > You can tolerate a couple of prism diopters of vertical displacement without > problem. One can tolerate 2 prism diopters of vertical displacement? I thought you said 1.5 prism diopters vertical displacement can already cause headache. I know we have fusion reserves. But why stretch the reserves when one can get a pair of glasses with dead center optical centers? > > However > > you hint that in the US. Eyeglasses are accurately made so I [quoted text clipped - 6 lines] > > -MT I won't mind even if I have to pay ten times as long as I get it. Haven't you sent any glasses outside the country? I could give you the power (no astigmatism), PD (left from nose bridge center, right from nose bridge center), then you can choose the frame of whatever kind that is small, round, close to eyes and then fabricate it with MilSpec quality (zero displacement vertically and horizontally) and then send it to me after paying then I'll treasure your spectacle forever :) Horus > By putting covering each eye alternately with one hand, I can > see about 2mm vertical jump when using the glasses. No > vertical jump in contact lens or just 0.05mm. 2 mm vertical jump is meaningless if we don't know how far away you're measuring 2 mm. At 40 cm, a 2 mm vertical jump works out to about 0.4 prism diopter. At 6 m, a 2 mm vertical jump would be difficult to detect. > Well. I have 4 pairs of glasses. One has horizontal displacement > of 5mm, one by 3mm, one by 2mm, one also by 2mm. We don't know whether you mean displacement of the optical centers, or phoria. If you mean phoria, you have to tell us how far away you're measuring that 5mm or 3mm. > Vertical is > off by 1mm. Do you mean 0.25 prism diopters? That's a 1 mm jump at 16" from your eye.. > Now I want glasses that is dead center vertically > and horizontally. Its ok if I have very slight phoria but at least I > want my glasses to be accurate so the phoria won't be from > many sources. > Say. Have you encountered eyeglasses frames that have collimination > screws that hold the lens? This means the lens are not stuck to the > frame body but by the collimination screws entirely holding them > (floating the lens just like in SCTs telescope)? http://www.nealjguns.com/ss_store/ShootingGlasses.html or maybe http://www.optivision2020.com/trial-lens-sets.html This is the ultimate in frame design and centering. I think you should find an experienced optician and ask him to make you a good pair of glasses and relax and let him do his job. > is zero then prestiss rule = power x zero = zero. In this case, > how do you solve for the vertical prism diopter of the -7 and -8 > when there is no displacement in distance but by power?? In most cases you don't read through the optical centers. If you read 10 mm below OC in each eye and the glasses are made "perfect", your -7 eye will experience 7 pd base down and your -8 eye will experience 8 pd base down when reading 10 mm below OC. The difference is 1 pd. You probably have managed pretty well. If you want to do better, have the optician put the OCs _low_ in the frame, or whereever you happen to be looking when you read. Then you might experience vertical diplopia when looking up, but in down gaze the glasses should be very comfortable. > One can tolerate 2 prism diopters of vertical displacement? I thought > you said 1.5 prism diopters vertical displacement can already cause > headache. It's silly to think everyone is the same. 1.5 vs 2 but what does it matter? Bottom line: do you have vertical phoria when you read? > I know we have fusion reserves. But why stretch the reserves when > one can get a pair of glasses with dead center optical centers? Reason number one: you don't read through the optical centers. > and then send it to me after paying then I'll treasure your spectacle > forever :) Sorry, I'm not in that business. -MT > > By putting covering each eye alternately with one hand, I can > > see about 2mm vertical jump when using the glasses. No [quoted text clipped - 18 lines] > > Do you mean 0.25 prism diopters? That's a 1 mm jump at 16" from your eye.. I mean displacements of the optical centers. With the -7.0D power, it translates to about -7.0 x 2mm = 1.4 diopters. It may be within the fusion reserves of the eyes but 0 prism diopters would be optimum. Anyway, thanks for your superb technical assistance. I'll let them fabricate more pairs, so statistics says out of 5, 1 will be superb. And if it won't happen, then I'll learn eyeglass making myself and just handle the machine directly. Also I talked to one of the boss who demo me the machine and I discuss stuff with the optician. I'll be there when he fabricates my glass watching every step of the way. Thanks again. You are a Godsend amidst a world of darkness and ocular ignorance. Hope they nominate you for US Presidency someday. Horus > > Now I want glasses that is dead center vertically > > and horizontally. Its ok if I have very slight phoria but at least I [quoted text clipped - 47 lines] > > -MT > Hope they nominate > you for US Presidency someday. > > Horus That's if there *is* such a thing as a President (or a US for that matter) after the current one and his cronies get done socializing us. :( -=# Firewalker #=- > > Hope they nominate > > you for US Presidency someday. [quoted text clipped - 6 lines] > > -=# Firewalker #=- While we have sporadic issues with Otis, s.m.v. stays BLISSFULLY free of political crap. Please keep YOURS to yourself. Thanks :-) > Convergence is normally inseparable from accommodation. If you accommodate, > you converge. If you converge, you accommodate. If you alter one side of > that relationship without considering the other it's like running a bicycle > with wheels of different sizes. Not a good example, perhaps. This is an ordinary bicycle: http://en.wikipedia.org/wiki/File:Ordinary_bicycle01.jpg That's the name, not the description. SignatureDan Abel Petaluma, California USA dabel@sonic.net >> > In prescribing glasses for myopes. The prescription says corrected >> > for distance and not near. [quoted text clipped - 9 lines] >make sense to get a 65 (corrected for near) pair rather than >corrected for far (68) if my PD is 68? Get contacts. Problem (however imaginary) solved. Signature- Mike Ignore the Python in me to send e-mail. |
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