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Medical Forum / General / Vision / February 2006Astigmatism vs. distance
Does the required power/axis generally change vs. distance? If so, I assume common practice is to prescribe for distance and take what you get for close? Not sure about progressives, but in bifocals it seems like it would be simple enought to change cylinder power for close. Is it normally done? I'm wondering if there are possibly undesirable side effects to having too much cylinder power while doing long hours of close work. I self prescribed myself some $20 internet computer glasses by just adding +1 to my doc's Rx and putting a light amber tint. While I was at it I reduced the cylinder by 0.25 because I suspected that I needed less cylinder at close. I just got them a few days ago and I'm very happy with how they work (although you get what you pay for; the frames are on par with $5 drug store sunglasses). I realize it's all very non-scientific, since I can't compare the reduced cylinder to my distance glasses (I can't see distance with the extra +1). Maybe I have more than I need in the first place. On 2/19/06 8:44 AM, in article Kf1Kf.565337$084.125206@attbi_s22, "Charles" <nospam@nospam.com> wrote: > Does the required power/axis generally change vs. distance? If so, I > assume common practice is to prescribe for distance and take what you [quoted text clipped - 14 lines] > reduced cylinder to my distance glasses (I can't see distance with the > extra +1). Maybe I have more than I need in the first place. I am not medically trained. Nevertheless, I have enough experience to realize that very little in medicine depends on what us dumb engineers call high precision. The most recent important case where it made a difference was in the recent hunting accident involving our wonderful vice president. A pellet cause heart problems in which a slight difference in location might have meant a much graver situation. Even there, there was no heart penetration so precise absolute measurement is of little value. It is the relative measurement that is significant. Getting back to vision, most optometric lenses are manufactured in steps of 0.25 D(iopter). I doubt that even the best diligent optometrist will be able to determine the optimum correction with an error of 0.125D or less. Moreover, I believe the party line is that the crystalline lens only changes spherical focal length when it accommodates. I am pretty sure that optometrists on this newsgroup will be aware of unusual eye malformations that will be exceptions. The bottom line is that I would not expect change of cylinder (astigmatism) with viewing distance to be a problem compared to all the other things likely to give problems. Bill -- Ferme le Bush > I am not medically trained. Nevertheless, I have enough experience to > realize that very little in medicine depends on what us dumb engineers call [quoted text clipped - 4 lines] > penetration so precise absolute measurement is of little value. It is the > relative measurement that is significant. It doesn't have to penetrate the heart muscle to cause problems. It could jam into the AV node to set off an electrical problem or a blood vessel to wreck havoc there. He might be *your* vice president, but not other readers here and the issue of whether his is "wonderful" might be countered by a discussion about how he is far from that. But I digress. > Getting back to vision, That's a good idea considering that this is a vision newsgroup. >most optometric lenses are manufactured in steps of > 0.25 D(iopter). Lenses can be fabricated in any power. >I doubt that even the best diligent optometrist will be able > to determine the optimum correction with an error of 0.125D or less. Well I do it all the time. I even calculate final contact lens powers using a program that determines the resultant power to 0.1D, and I have labs that make lenses to that tolerance.. The reality is that the majority of people are not sensitive to .12D variance. Personally, I am and when I find a patient who is so, I am as diligent as possible to assure that s/he gets that final power. > Moreover, I believe the party line is that the crystalline lens only changes > spherical focal length when it accommodates. I am pretty sure that [quoted text clipped - 4 lines] > with viewing distance to be a problem compared to all the other things > likely to give problems. You might not but the reality is that for some people astigmatism does change as the lens accommodates, hence the reason for checking cyl at near and distance separately. But there is one point that you are correct: the effects of pupil size, cornea aberrations, optical imbalance, etc are more likely issues than changes in cyl. LB, O.D. > Does the required power/axis generally change vs. distance? If so, I > assume common practice is to prescribe for distance and take what you [quoted text clipped - 14 lines] > reduced cylinder to my distance glasses (I can't see distance with the > extra +1). Maybe I have more than I need in the first place. There is something called meridional accommodation which theoretically can mean a different cyl measurement for near than for distance. But in practice I don't know that very many optometrists would routinely test for cyl separately at near. Generally when prescribing cyl you are conservative and err on the side of less cyl anyway. In the case of your experiment, the extra +1 would make much more difference to your visual comfort than the 025 reduction in cyl. Dom Great, the engineers are self prescribing over the net. I may s well not go to work today and ask an engineer to take over for me. It is a well known fact that engineers know more than optometrists, that is why there are so many of them. On 2/20/06 11:01 AM, in article 1140461997.997635.172170@g43g2000cwa.googlegroups.com, "CatmanX" <drgrant@ozemail.com.au> wrote: > Great, the engineers are self prescribing over the net. I may s well > not go to work today and ask an engineer to take over for me. It is a > well known fact that engineers know more than optometrists, that is why > there are so many of them. If you are referring to me I defend myself by saying optometrists know more about certain aspects of optics while optical engineers know more about other aspects of optics. Even the best optometrist cannot change the laws of physics and chemistry by lobbying their licensing boards. A good optometrist would not attempt to do so. Bill -- Ferme le Bush > Great, the engineers are self prescribing over the net. I may s well > not go to work today and ask an engineer to take over for me. It is a > well known fact that engineers know more than optometrists, that is > why there are so many of them. Geez man, what's your deal? Would you be less upset if I was a plumber? Do I really need to talk to the optometrist about reading glasses when I can just copy the near Rx from my progressives and save over $200 on the deal? On 2/19/06 9:12 PM, in article 43F94FC0.5080207@spam.me, "Dom" <dont@spam.me> wrote: > There is something called meridional accommodation which theoretically > can mean a different cyl measurement for near than for distance. But in > practice I don't know that very many optometrists would routinely test > for cyl separately at near. Generally when prescribing cyl you are > conservative and err on the side of less cyl anyway. Because cylindrical correction is not uniquely defined, what does "less cylinder" mean? From reading of this newsgroup, I understand that optometrists and ophthalmologists tend to disagree on how the convention to prescribe cylinder. I like Zernicke functions because they are orthonormal and remove that ambiguity. Bill -- Ferme le Bush > Because cylindrical correction is not uniquely defined, what does "less > cylinder" mean? From reading of this newsgroup, I understand that > optometrists and ophthalmologists tend to disagree on how the convention > to > prescribe cylinder. Why is it ambiguous to you? The two notations describe the very same surface. > I like Zernicke functions because they are orthonormal > and remove that ambiguity. Try to get an optician to make glasses from Zernicke terms. Better still, try to find one that can spell "Zernicke". -MT On 2/20/06 4:00 PM, in article HKsKf.1057$F56.1016@newsread3.news.atl.earthlink.net, "Mike Tyner" <mtyner@mindspring.com> wrote: >> Because cylindrical correction is not uniquely defined, what does "less >> cylinder" mean? From reading of this newsgroup, I understand that [quoted text clipped - 13 lines] > > -MT While the prescriptions describe the same piece of glass, they are written in different ways. It is analogous to describing something in English or in French. The prescriptions look superficially different. So I again ask the question: what corresponds to minimum cylinder when you specify the axis at 90° from what someone else specifies as the axis? The Zernicke business goes to show that a dispensing optician does not have to be very knowledgeable of other fields of optics. That is OK with me. They do not need to know. Bill -- Ferme le Bush > So I again ask the question: what corresponds to minimum cylinder when you > specify the axis at 90° from what someone else specifies as the axis? The cylinder is constant whether you list the steepest meridian first or last. Previously, you asked >Because cylindrical correction is not uniquely defined, what does "less > cylinder" mean? "Less cylinder" means less difference between the steepest and flattest meridians. -MT On 2/20/06 10:03 PM, in article 63yKf.1183$F56.16@newsread3.news.atl.earthlink.net, "Mike Tyner" <mtyner@mindspring.com> wrote: >> So I again ask the question: what corresponds to minimum cylinder when you >> specify the axis at 90° from what someone else specifies as the axis? > > The cylinder is constant whether you list the steepest meridian first or > last. Except for sign. So I presume that minimum cylinder means minimum absolute cylinder. And absolute value of cylinder is uniquely defined. I am really surprised that you get into the differential geometry of optical surfaces. > Previously, you asked > [quoted text clipped - 5 lines] > > -MT -- Ferme le Bush > Except for sign. So I presume that minimum cylinder means minimum absolute > cylinder. And absolute value of cylinder is uniquely defined. Sure. In the industry, plus and minus cylinder form are so commonly interchanged that absolute value is understood, just as absolute value is assumed when "less correction" can mean "less plus" or "less minus." > I am really surprised that you get into the differential geometry of > optical > surfaces. If you mean the Zernicke polynomial, I try not to. But it's a prerequisite for understanding refractive surgeons, topography and wavefront optics. -MT On 2/21/06 10:58 PM, in article uYTKf.2010$S25.1491@newsread1.news.atl.earthlink.net, "Mike Tyner" <mtyner@mindspring.com> wrote: >> I am really surprised that you get into the differential geometry of >> optical >> surfaces. > > If you mean the Zernicke polynomial, I try not to. But it's a prerequisite > for understanding refractive surgeons, topography and wavefront optics. By differential geometry, I mean the study of curves and surfaces based upon local derivatives. In the situation under discussion you are looking for the best local fit using cylindrical plus spherical terms only. You leave out the xy terms that lead to the Zernicke hyperbolic terms. Bill -- Ferme le Bush > On 2/19/06 9:12 PM, in article 43F94FC0.5080207@spam.me, "Dom" > <dont@spam.me> wrote: [quoted text clipped - 13 lines] > Bill > -- Ferme le Bush Less cylinder meaning closer to zero cylinder. i.e. +1.25 cylinder is less than +1.50 cylinder. And -1.25 cylinder is less than -1.50 cylinder. Dom |
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