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Medical Forum / General / Vision / October 2006difficulty of making small corrections for distance vision
My vision has finally deteriorated to the point where I need glasses for both near and distance. My problems are minor compared to those of many people; yet the experience so far has been frustrating. I got prescription glasses for distance. But distant objects (such as birds) were still a bit blurry. After a month, I went back to my regular doc (he'd been on vacation when I got the first exam). He rechecked my eyes and found 1/4 diopter of astigmatism in one eye. Things are slightly out of focus through my other eye, too, but I don't know why. He'll check my eyes one last time before I get the scrip filled. I'm concerned we may miss something. He said my astigmatism was so minor as to hardly exist, and, compared to the serious medical problems of his other patients, I'm sure this is true. But *I* can sure see the blurriness! It really takes the pleasure out of birdwatching, which is my main recreation and stress relief. Questions: The "distance" part of the eye exam is done at only 20 feet. I'd think that if you wanted your focus to be tack-on at 150 feet instead (given that you can't have both), you'd test it with something further away. Is there such a thing? Also, the letters used for the exam are projected from an overhead projector. They aren't that sharp to begin with. Most real objects have a clearly defined edge, but these don't. Thus, it's hard to tell, when looking through the exam machine, whether "this one or that one" is better. But you sure can tell outdoors if something's not sharp. Does anyone ever test distance vision using real objects - letters on a sign, or something? Also, it seems the test lenses do mostly gross corrections. If you have only a little bit of astigmatism or prism or something, I'm thinking maybe the examiners can't always find that, because their tiniest correction is still way bigger than what you need. True? I'm pessimistic that we'll be able to correct everything that's wrong. I'm also not sure how motivated the doc will be to fix the scrip from "good enough" to "as good as possible". What can I do at this upcoming exam to get the best results? thanks... LD central USA > The "distance" part of the eye exam is done at only 20 feet. I'd think > that if you wanted your focus to be tack-on at 150 feet instead (given > that you can't have both), you'd test it with something further away. > > Is there such a thing? Yes, but the optical difference between 20 feet and 150 feet is about an eighth of a diopter. > Also, the letters used for the exam are projected from an overhead > projector. They aren't that sharp to begin with. Most real objects > have a clearly defined edge, but these don't. Normally the projected image has edges that are quite sharp. If it wasn't sharp, then it was poorly focused, or poorly cleaned. > Thus, it's hard to tell, > when looking through the exam machine, whether "this one or that one" > is better. The point where you can no longer tell a difference is an important endpoint. ("Less is worse, but more is not better.") > But you sure can tell outdoors if something's not sharp. > Does anyone ever test distance vision using real objects - letters on a > sign, or something? As long as the final testing is done binocularly, there's exquisite correlation between the diopter values you choose in the refracting lane and the values you'd choose in real space. Depending on age and occupation and the purpose of the glasses, there are conventions for rounding and shaving the prescription for the sake of comfort or acuity at different distances. > Also, it seems the test lenses do mostly gross corrections. If you > have only a little bit of astigmatism or prism or something, I'm > thinking maybe the examiners can't always find that, because their > tiniest correction is still way bigger than what you need. > True? Very few doctors would prescribe 0.12 diopters of astigmatism, for three reasons. Most people couldn't appreciate it. Many labs would have trouble reproducing 0.12 accurately. Finally, cycles of hydration, oxygenation and circulation often cause 0.25 or more variance morning-to-night. > What can I do at this upcoming exam to get the best results? Understand that the quality of the chart image doesn't necessarily interfere with your choosing the best lenses. The doctor wants to know whether a) one is better than the other or b) there is no difference. He will stop at the point where your answers change from a) to b). Then he (or she) should back up a step to be sure that "less is worse." The test for astigmatism can be distorted by using certain letters to judge which is "less blurry." E's, M's, T's etc can be misleading. Pick a round letter to use during the "JCC" test, preferably a big "O". Just my opinion. But then I'm the only one in the world who knows how to refract. :) -MT > > The "distance" part of the eye exam is done at only 20 feet. I'd think > > that if you wanted your focus to be tack-on at 150 feet instead (given [quoted text clipped - 3 lines] > > Yes, What does it involve? My curiosity is killing me now. >but the optical difference between 20 feet and 150 feet is about an > eighth of a diopter. Is that a visible difference? I'd rather the distant stuff be clear, at the expense of the closer stuff. > > Also, the letters used for the exam are projected from an overhead > > projector. They aren't that sharp to begin with. > > ... If it wasn't > sharp, then it was poorly focused, or poorly cleaned. I dunno how clear it's supposed to be. I got up on a chair and looked at them on the wall. They have jagged edges up close; at a distance these edges look soft or fuzzy. This is normal for an image projected from a xeroxed transparency, but far from the crispness of a piece of paper with letters printed on it. > > Thus, it's hard to tell, > > when looking through the exam machine, whether "this one or that one" [quoted text clipped - 3 lines] > endpoint. > As long as the final testing is done binocularly (Please say more about that...) > there's exquisite > correlation between the diopter values you choose in the refracting lane and > the values you'd choose in real space. Well.... I guess I'm skeptical because of my experience with it so far. The first optometrist to examine my eyes (seen because my regular doctor was out of town) did this test. When we got to that point, and I said, "A is a little better than B, but both are still out of focus", she said, "That's as good as it gets". I suggested there might be astigmatism, and she put up a lens that when I looked through it made things *totally* out of whack. So she said I didn't have astigmatism at all. Since doc #2 says I do and she says I don't, this causes me to think that maybe this test is not that precise. It might be the person administering it, but I think the test images and/or test lenses also make it harder for the patient to give reliable answers. >Depending on age and occupation and > the purpose of the glasses, there are conventions for rounding and shaving > the prescription for the sake of comfort or acuity at different distances. OK. > > Also, it seems the test lenses do mostly gross corrections. If you > > have only a little bit of astigmatism or prism or something, I'm [quoted text clipped - 4 lines] > Very few doctors would prescribe 0.12 diopters of astigmatism, for three > reasons. Most people couldn't appreciate it. Yes, I get this impression too, that many people don't seem to notice even rather large haloes, or misalignment, or whatever. Unfortunately, I can indeed detect small differences in clarity. When birding, you're often trying to spot something very small from quite some distance away. Any deterioration of this capacity is noticeable, and frustrating. (I'm probably spoiled, also, by having spent so many years studying fine detail through very nice binoculars.) >Many labs would have trouble > reproducing 0.12 accurately. Finally, cycles of hydration, oxygenation and > circulation often cause 0.25 or more variance morning-to-night. Yeah, I don't know what 0.12 diopters would even look like. What IS the smallest correction they have in that arsenal of test lenses, anyway? > > What can I do at this upcoming exam to get the best results? > [quoted text clipped - 3 lines] > point where your answers change from a) to b). Then he (or she) should back > up a step to be sure that "less is worse." > The test for astigmatism can be distorted by using certain letters to judge > which is "less blurry." E's, M's, T's etc can be misleading. Pick a round > letter to use during the "JCC" test, preferably a big "O". Can you say more about this? I thought that astigmatism was manifested by thin lines looking double at one angle and OK at another angle. So I would have tried to use E, M, and T. The Os are better? What is "JCC"? I'm afraid I'm going to go in there, do whatever can be done, and still come out with glasses that leave much to be desired. thanks, LD Indianapolis central USA Home of the dull-colored warbler, the tiny cryptic sparrow, and many birders with very expensive binoculars..... *sigh* I sympathize with your desire to optimize your vision. You should certainly shop around for an eye doctor who sympathizes as well. That would be my first suggestion. My previous eye doc wanted to blow off 0.25 diopter of correction, which I found to make an absolutely enormous difference. If you are like me, being 0.25 nearsighted relative to what you are used to is totally unacceptable. Having said that, I doubt going in increments smaller than 0.25 is worth the trouble. Remember that your eyes can accomodate to near, so you can overshoot the minus power by a smidge and you'll pull it in when looking far away. Astigmatism doesn't work that way, but my experience is that you simply can't notice errors smaller than 0.25. And I've been called picky. If you want to be anal and really understand what's going on (like me) you can go out on E-bay and get yourself a cheap trial lens kit to experiment with. I found it very educational, and you can hold lenses up over your glasses or contacts to second guess your Rx in real life situtations (like looking at birds?) Have you considered RGP contacts? I recently changed, and the vision, especially distance, is the best I've ever had. It took a few doctors to get it right though, so I'd recommend seeking out someone who has a fair number of RGP patients. (I'm a patient, not a doctor) -- > [ ... ] > Have you considered RGP contacts? I recently changed, and the vision, > especially distance, is the best I've ever had. It took a few doctors > to get it right though, so I'd recommend seeking out someone who has a > fair number of RGP patients. I don't think that contacts are the answer for not being able to get a proper script for spectacles. My experience is that approximately one out of each three alleged refractometrists are able to get to within 0.5D of one's actual need for both of two eyes at one sitting. The really bad ones may get bristly if challenged, like when you go back with your new glasses and claim blur. > If you want to be anal and really understand what's going on (like me) > you can go out on E-bay and get yourself a cheap trial lens kit to > experiment with. I found it very educational, and you can hold lenses > up over your glasses or contacts to second guess your Rx in real life > situtations (like looking at birds?) Yes, I agree that anality is the best answer here. I wish I knew more about cheap trial lens kits. (I have got some, but they were not particularly cheap). Having a test pattern for astigmatism on a wall can be useful also. For me, the final test is reading lit signs at night, in the distance. In the end, one must master the simple arithmetic of diopters which is sometimes known to the phoropter jockeys (who, in my experience, are sometimes, if not frequently, on-the-job-trained lackeys). -- Dicky (unprofessional) >> > Is there such a thing? > What does it involve? My curiosity is killing me now. In my case it consists of "verifying" the endpoint, outside the exam room in free space, using handheld lenses held over a trial frame or the patient's old glasses. But I can't see where free-space targets would be better for astimatism or balance. The exam lane is the best place to measure astigmatism accurately, and to determine the optical difference between the eyes. Those can be found at most any distance. Clean charts do help, but they aren't crucial. If a short testing lane causes any problems, it's almost always a little too plus in one or both eyes, and therefore a little blurry at real infinity. > Is that a visible difference? > I'd rather the distant stuff be clear, at the expense of the closer > stuff. Knowing that, if you're 30 or under, I might write an extra quarter of minus in each eye but I'd warn about headaches and difficulty reading. > I dunno how clear it's supposed to be. I got up on a chair and looked > at them on the wall. They have jagged edges up close; at a distance > these edges look soft or fuzzy. This is normal for an image projected > from a xeroxed transparency, but far from the crispness of a piece of > paper with letters printed on it. That's not typical. Mine are smooth, just a little blurring at the limits of resolution for projection film. >> As long as the final testing is done binocularly > > (Please say more about that...) The last few steps are usually done with both eyes open, because your focusing reflexes are prone to change when one eye is closed. If the refraction is properly balanced, a quarter diopter less minus will make things worse, by an equal amount in each eye. But those last refinements assume that astigmatism has been corrected accurately, and that's where I think you've gotten short shrift from your refractionists. > she said, "That's as good as it gets". I suggested there might be > astigmatism, and she put up a lens that when I looked through it made > things *totally* out of whack. So she said I didn't have astigmatism > at all. Well, the lens used to test astigmatism _should_ make things a little blurry, but not worse than 20/30 or 20/40. It's presented pairwise, 1 and 2, and either 1 is better, or 2 is better, or you can't tell a difference. "Can't tell a difference" is often the endpoint. If she only showed you _one_ lens when testing for astigmatism, find another doctor. To be able to say "there isn't any" you should be presented with at least 4 choices, unless there's some pre-testing that tells the doctor which quadrant to search. > Since doc #2 says I do and she says I don't, this causes me to > think that maybe this test is not that precise. It can be, but it takes more time and attention than many can give, and it's tedious and many patients lose track and start answering at random. That's always fun. :( > Yeah, I don't know what 0.12 diopters would even look like. What IS > the smallest correction they have in that arsenal of test lenses, > anyway? Phoroptors (the 1/2 thing) have provision for 0.125 lenses but it's cumbersome and seldom used. Hand-held lens sets may also have 0.12 lenses but they'll be dusty and unused in most offices. > Can you say more about this? I thought that astigmatism was > manifested by thin lines looking double at one angle and OK at another > angle. You're right about the angles. But mathematically, astigmatism shouldn't create double images. In real humans, it often does. In the astigmatism test, it's overall blur you should consider, rather than "doubleness". Sometimes "doubled" images can't be corrected with glasses, but RGP contacts can be spectacular. > So I would have tried to use E, M, and T. The Os are better? Absolutely. If I had one, I'd use a chart with nothing but Os of different sizes for this test. > What is "JCC"? That's the test for astigmatism we've been talking about. It's a way of using cylinder lenses (cylinder=astigmatism) to determine both the amount and the axis of astigmatism. Google "Jackson Cross Cylinder". > I'm afraid I'm going to go in there, do whatever can be done, and still > come out with glasses that leave much to be desired. I'd shop for another doctor. I'd call a few offices and tell them you've had trouble getting a good refraction, and ask which of their doctors have the best luck satisfying engineers. (Not a dispersion on engineers, but they do have a reputation among optometrists for demanding very precise refractions.) Call several of these offices and see who gives you the most satisfying answer. It's possible that glasses won't give you the sharpness you desire. In some of those cases, RGP contacts can be spectacular. -MT [Charles] I sympathize with your desire to optimize your vision. You should certainly shop around for an eye doctor who sympathizes as well. [Mike Tyner] I'd call a few offices and tell them you've had trouble getting a good refraction, and ask which of their doctors have the best luck satisfying engineers. Ah HAH. Thanks for this tip. [Dick Adams] My experience is that approximately one out of each three alleged refractometrists are able to get to within 0.5D of one's actual need for both of two eyes at one sitting. The really bad ones may get bristly if challenged, like when you go back with your new glasses and claim blur. Yes, that's what happened with the first one. :-( I don't want to be a pain, but I would prefer to nail down the refraction during the exam, even if that's tedious - rather than nail it down by ordering glasses and looking through them. If the person does a good exam, and finds that I have between zero and 1/8 diopter of whatever, and there's no realistic way to correct for that, I'll live. But I'd like to be confident that we did find out just what's wrong with my vision. I'd like to not have to keep wondering whether if we tried the exam yet AGAIN it would help. The current person is an ophthalmologist, and he examined my eyes for disease etc. as well. I think he is reasonably competent and patient, but I worry that just by the nature of the test (especially those fuzzy letters) and the natural flakiness of vision (it seems you need to look back and forth at things a couple of times to be sure you're on target), that it might take more than "reasonably". :-( =============== [LD] > [The distance charts] have jagged edges up close; at a distance these edges look soft or fuzzy. This is normal for an image projected from a xeroxed transparency..... [Mike Tyner] That's not typical. Mine are smooth, just a little blurring at the limits of resolution for projection film. Oh dear. I've never seen yours, so I can't tell if those I've seen are any worse. Maybe they look find to most people and I'm just picky. But the letters have never looked really sharp to me. They're dimly lit, too. I feel as though my ability to accurately make fine discriminations with them is limited. It's not like the close-up part of the exam, where there is little doubt as to whether what you're seeing is clear. Why use a projector at all? Wouldn't it also work to just put a piece of paper on the wall, in the same place, with the letters on it the same size as they are projected on the wall? You'd see the same thing in the mirror as you do now, and you could put a lot brighter light on it too. Right? I imagine the doc would have a cow if I brought such a thing in. =================== [much info on how astigmatism tests work and what to look for] Thank you all, this is very helpful. Phew. [Mike Tyner] The lens used to test astigmatism _should_ make things a little blurry.... In other words, you make it too blurry - you overshoot - and then work backwards to where it looks right? Also: My existing glasses correct for one eye being different from the other. (Distance correct ion = +1.25 right eye, +1.75 left eye). Will it help to wear these glasses before I go in? I'm thinking that my eyes will have time to adjust to roughly where they need to be, before trying to fine-tune the more difficult things. ? =================== [various] ....RGP contacts can be spectacular. Thanks... I read about them (there is a good web site at http://www.allaboutvision.com/contacts/rgps.htm But my initial reaction is EEEEEEEEEEKKK you put something in your EYE!!!! Even seeing people wearing contacts gives me the crawls. The description sounds like they might give really good vision, but I dunno if I'm ready for them. ================== [CC] Viewing birds at a distance? It's difficult. You have to locate the bird in the landscape first, with just your glasses, before you can put binoculars on it. But the birds try to stay invisible. Lately, I've been watching shorebirds, because they tend to stand still out in the open, rather than rushing behind a leaf. With birds in trees, you have to be fast, or all you get is a glimpse of something's tail 2 seconds before it vanishes. I can tell you of many fine tail views, or at least partial views, or at least leaves that were almost partial views..... :-) cheers, Liz D. One of the problems with being an optometrist is that we do test in unnatural surroundings. The peripheral vision is the major component in focussing, but we block it out behind a refractor. We assume that 6m is infinity, when it clearly isn't. As a result, a percentage of people tend to get mildly overplussed, resulting in fractional blur at distance. How do we eliminate this? Longer rooms, trial frames, finishing off in the waiting room while you look across the road, I am about to install a wall sized mirror to reduce the effect of a wall 4m away. I would recommend you get retested in a trial frame, then finish off the final spherical component out of the consulting room. dr grant > One of the problems with being an optometrist is that we do test in > unnatural surroundings. The peripheral vision is the major component in > focussing (It is? I didn't know that.) >but we block it out behind a refractor. We assume that 6m is > infinity, when it clearly isn't. [quoted text clipped - 7 lines] > I would recommend you get retested in a trial frame, then finish off > the final spherical component out of the consulting room. I've not heard of a trial frame, or of any testing being done outside the exam room. It sounds like a smart idea, but how would you get the exam lenses out into the waiting room? (Remember, I don't know what most of this stuff looks like.) It does seem like it would be a bit more reliable to double-check the scrip by looking at real objects that have real edges under good outdoor light. I didn't realize you could do that without actually having glasses made. LD CatmanX schreef: We assume that 6m is > infinity, when it clearly isn't. Add a - 1/8 dpt and you are done (I take it for granted you did know already..) A question related to this, do you use a red/green test? Jan (normally Dutch spoken) Adding -.125D can be misleadsing also. What we find in a refraction is not necessarily what the eye wants in the real world. As much as most (99%) of refractions are fine with what we find in the refractor head, there is this small group that are probably active accommodators that give funny results with their new specs. And no, I don't use red/green, I can't do it (brain wiring probably) and don't ask it of my ptients as I can't adequately explain what they are supposed to get. I always finish with a frial frame and +/-0.25 flippers. grant (Sorry for posting to the group, but I can't get through to you via private mail.) thanks, Liz D. > The test for astigmatism can be distorted by using certain letters to judge > which is "less blurry." E's, M's, T's etc can be misleading. Pick a round > letter to use during the "JCC" test, preferably a big "O". I have oblique axis astigmatism in both eyes, not much--.5 D I think, but you're right--only the big "O" reveals it. I asked my optometrist why doctors don't use the "fan spokes" image to test, and she said it's old fashioned, not used anymore. Why is that? With the big "O" I have to look at specific parts of it--the diagonal parts--and try to explain which are blurry. My eye doctor had me turn the axis slider on the phoropter (?) lenses to try to get the axis correct. This was minimally effective. I gave up getting the astigmatism correction in my glasses--there were just too many variables--the Rx never seemed right. Partly it's the type of lens, the shape of the frame, the accuracy of how the lens is placed in the frame, the way the frame sits on my assymetrical nose and ears... And then my eyedoctor told me that the cylinder correction in the lens only "corrects" about 15 degrees of arc, whereas, I perceive a greater arc than that in my blurry astigmatism axis. Is this correct, or have I misunderstood? But viewing birds at a distance? For me, without binoculars--only glasses, that would be impossible. And with my contacts, forget it. Thanks, C66 |
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