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Medical Forum / General / Vision / October 2004Smaller glasses blurry off center
I recently got a new pair of glasses. My old ones are many years old, and as was normal at the time, a lot bigger than what is common today. The newer ones are significantly smaller. I find that when I look at objects significantly off of center, it is very blurry. The blurriness starts around 45 degrees off of center. This is really disappointing. I've seen pictures of what things look like from the point of view of someone suffering from eye disease, and it kind of looks like that (clear in the center, blurry off of center). Probably no as severe, of course. But I'm not sure it is something that I want to get use to. The technician says that it is a natural consequence of the smaller glasses, as well as edge distortion from the high index glass. She says that the blurry off-centerness would not be so severe for low index glass. Is this in fact the case? Is what I described pretty typical? Fred >I recently got a new pair of glasses. My old ones are many >years old, and as was normal at the time, a lot bigger than [quoted text clipped - 10 lines] >The technician says that it is a natural consequence of the >smaller glasses, Not true. >as well as edge distortion from the high >index glass. She says that the blurry off-centerness would >not be so severe for low index glass. That's possible, depends on the Rx. There are other ways to blur the peripheral vision. Tell us your Rx, type of lens (single vision, multifocal etc), the index of refraction of the new and the old lenses, and the base curve if available. >Is this in fact the case? Is what I described pretty typical? > >Fred Hope this helps Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > >I recently got a new pair of glasses. My old ones are many > >years old, and as was normal at the time, a lot bigger than [quoted text clipped - 12 lines] > > Not true. Sort of what I thought. I don't have a background in optometry, but I have studied optics from an engineering perspective. > >as well as edge distortion from the high > >index glass. She says that the blurry off-centerness would [quoted text clipped - 3 lines] > vision. Tell us your Rx, type of lens (single vision, multifocal etc), the index > of refraction of the new and the old lenses, and the base curve if available. I'll give details for the left lens, which is the one with the peripheral blurring. According to the optician, the old glasses have -4.75sph (I think it stands for "sphere") and the new glasses have -5.75sph. There are also spaces on the prescription for "cylinder", "axis", "prism", and "add", but they are empty for the left lens. All the lenses are single vision. The new glasses actually use high index plastic, 1.67. They claimed that the old one is also high index plastic, maybe 1.6. Funny thing is that I was under the impression that I bought it as high index glass, many many years ago -- with AR coating, though the current optician says it doesn't have any. That's a bit of a bummer, because the motivating factor for me to get new glasses is because I scratched the old ones in the center of view. This might not have happened if it has AR coating, since it also acts as a scratch-resistant coating. Anyway, the base curve of the new glasses is +2.00. On the old glasses, it is +4.00. I also had the old glasses measured elsewhere, which gave a base curve of +3.25. I'm not sure whether the discrepancy between +3.25 and +4.00 for the old glasses is significant. That other place also found that the new glasses has "spherical aberration" in the area where I saw peripheral blurring. According to my optician, though, this has limited meaning in a retail setting, where most places do not have the means to measure it. It also wasn't clear to her what the other place meant by spherical aberration, though admittedly, a technical, unambiguous definition exists. My optician's lab guy also claims that all lenses have spherical aberration. Yet another discrepancy came up regarding info on my new glasses. I had requested high index glass. According to my optometrist, it was actually made with high-index plastic. When I brought it elsewhere to check for problems, they claimed that it was definitely glass. When I brought it back to my optician, she was adamant that it was high index plastic. She explained that they decided to use plastic because it was safer, cheaper, and pretty high index. I should add that the new glasses were apparently aspherical, while the older ones were not aspherical ("spherical"?). The strategy to correct the peripheral blurring is to make the new glasses with as similar characterisitcs to the old glasses as possible, including the use of similar index plastic, spherical design, and similar front curvature. They explained that aspherical is better, but was meant to be customized for each client (I guess that means material, prescription, etc..). On the other hand, commercial processes use a generic method in the aspherical design, which can give undesired results. Fred Hi Fred, Given the info it is possible, that being a high index plastic lens in a shallow frame, that the problem is that the lens surface is being distorted by the frame. I have seen this several times. The lens is secured by the nasal and temporal edges of the lens but is not secured by the upper and lower edges and therefore the lens twists in the frame. On checking the lens powers on a lensmeter(focimeter) they are correct. However using a radius gauge for checking the lens curves they are not correct. The way to check this is measure the curves with the lenses in the frame and then take the lenses out and measure the curves after about 30mins if the curves have changed then distortion is present. See if this helps Regards Ian Hodgson - Isle o > > >I recently got a new pair of glasses. My old ones are many > > >years old, and as was normal at the time, a lot bigger than [quoted text clipped - 72 lines] > > Fred > Hi Fred, > [quoted text clipped - 8 lines] > frame and then take the lenses out and measure the curves after about 30mins > if the curves have changed then distortion is present. Hi, Ian, I will ask about that. Just wanted to clarify what you meant by a shallow frame. Do you mean that the rim is very planar, or that they are not very tall? If you mean planar, would the rationale be that the lens curves, so the rim must curve just to maintain contact at the edges? Fred > > > >I recently got a new pair of glasses. My old ones are many > > > >years old, and as was normal at the time, a lot bigger than [quoted text clipped - 75 lines] > > > > Fred > > Given the info it is possible, that being a high index plastic lens in a > > shallow frame, that the problem is that the lens surface is being distorted [quoted text clipped - 11 lines] > tall? If you mean planar, would the rationale be that the lens curves, so > the rim must curve just to maintain contact at the edges? I just asked the optician about this. She said that they do keep an eye out for that kind of thing, and it shows up quite noticably when they try to clamp the rim together and screw it together. Thanks. Fred Hi Fred, By a shallow frame I mean depth of top to bottme rim of 35mm or less. What happens is that when the lens is edged in a lowish powered (say less then about 5D in the higher powered meridian) then the resultant edge of the lens is so thin it doesn't locate and secure in the rim groove. Not to a later post unless they stress test the lens or use a radius gauge it does not show up readily on glazing. Regards Ian Hodgson - Isle of Man > Hi Fred, > By a shallow frame I mean depth of top to bottme rim of 35mm or less. What [quoted text clipped - 4 lines] > Not to a later post unless they stress test the lens or use a radius gauge > it does not show up readily on glazing. Hi, Ian, Thanks for clarifying. Since I don't have the frames on me right now (they are being fitted with new lenses), I don't know the exact height. From memory, it is quite conceivable that they are are less than 35mm height. That seems to be the modern style these days. I'll print out the details you provided to see how the optician takes it. I don't have the expertise and equipment to test it myself. There are a few things which hopefully indicate that this is not the problem. One is that my prescription is quite strong, so the lens is on the thick side (-5.75sph). This is probably outside of your "lowish powered" ballpark, right? As well, it is high index plastic, but at the low end of the high index materials (I think it's 1.7); I am told that there are even higher index materials. Another thing is that it occurs on one eye, the one with a stronger prescription, but not the right eye. I re-read your details and it seems to me that there are two effects that you could be referring to. One is that lense is secured on only two opposing edges, out of 4 possible edges. The lens is then able to shift position because of this. This seems to fit your description of the edge of the lens not fitting securely into the groove of the rim, particularly if the lens is thin. The free play allows the lens to shift around. I'll have to take a look at the final product (when I have access to it) to be able to relate the this shifting. On my old glasses, the lens material forms a ridge around the edge of the lens, apparently shaped to fit into the rim of the frames. When the screws on the outer edge of the rim is tightened, the entire rim of the frame wraps around the entire rim of the lens. Because of the ridged protrusion from the edge of the lens fits into the groove in the frame's rim, there are are no isolated anchoring points. The entire rim anchors the lens. I will look for this in the new lens. The other effect that I imagine based on your description is that the edge of the lens doesn't align well with the rim, possibly brought on by the fact that the upward force on the rim is only from the nose piece. The metal arm that hooks around the ear doesn't provide any force against gravity, it just prevents the rim from tilting forward and falling off the face. In fact, the gravity of those arms might provide downward force on the rim. Together with the upward force by the nose piece, this creates a sheer force on the rim, which is then transmitted to the lens, thus distorting its shape. I'll see if they can do the test you described, checking the shape of the lense in the frame, ideally after it's been on my face for a while, and then 30 minutes after the lens are removed from the frame. I'm not sure if they will do this, but hopefully, they will see your reasons after reading your description. Thanks again for the explanation and suggestion. Fred >> >I recently got a new pair of glasses. My old ones are many >> >years old, and as was normal at the time, a lot bigger than [quoted text clipped - 72 lines] > >Fred Fred, The base curve change sounds normal. They were probably off the shelve finished lenses which just about guarantees the proper base curve. The reason for the off-axis blur is due to "transverse chromatic aberration", possibly aggravated by less than optimal lens position. TCA is created by the prismatic effect when the gaze is off-axis. The formula for TCA is cF/V. Where c is the distance from the OC in centimeters, F is lens power and V is the Abbe number. .16 prism diopters is said to be enough to cause the loss of one line on the chart. You can see from this formula that higher lens powers and lower abbe numbers increase TCA. The lens power is pretty much written in stone which leaves the abbe number as the only variable left to modify, although increasing the accuracy of the OC (optical center) placement and minimizing the wearers vertex distance (distance of the lens to the eye) can make substantial improvements in this regard. Your Rx has increased in strength and the abbe decreased in value (1.6 abbe 36-42 depending on brand) to 32 for 1.67 index. It's clear that the next course of action is to refine the fit of the lens in front of the eyes and/or use a less dispersive lens material that has a higher abbe value. Hope this helps Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > The base curve change sounds normal. They were probably off the shelve finished > lenses which just about guarantees the proper base curve. The reason for the [quoted text clipped - 15 lines] > of action is to refine the fit of the lens in front of the eyes and/or use a > less dispersive lens material that has a higher abbe value. Hi, Rob, My experience with optics deals pretty well with numerical simulation of EM fields along wave guides. Much if imaging optics is outside of my experience. Would the above information be something that the optician and her lab guy knows about, or is it worthwhile trying to convey this to them by phone? They are quite a distance from me, so I don't think I'll be printing this out to show them. Fred >> The base curve change sounds normal. They were probably off the shelve finished >> lenses which just about guarantees the proper base curve. The reason for the [quoted text clipped - 26 lines] > >Fred Fred, First check the fit. Have an optician mark the optical centers horizontally and vertically. The OC should be on the center of the pupil for the horizontal and 2-5mm below the pupil on the distance gaze, with the lenses almost hitting the eyelashes. If the fit is ok, and the blurring is unacceptable, replace the lenses with a higher abbe value material. Seiko makes a nice 1.6 index aspheric lens that would almost be as thin as the 1.67 but with better off-axis optics. Good luck, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > I recently got a new pair of glasses. My old ones are many > years old, and as was normal at the time, a lot bigger than [quoted text clipped - 9 lines] > edge distortion from the high index glass. She says that the blurry > off-centerness would not be so severe for low index glass. Robert Martellaro wrote: > That's possible, depends on the Rx. There are other ways to blur the > peripheral vision. Tell us your Rx, type of lens (single vision, > multifocal etc), the index of refraction of the new and the old lenses, > and the base curve if available. > I'll give details for the left lens, which is the one with the peripheral > blurring. According to the optician, the old glasses have -4.75sph (I [quoted text clipped - 7 lines] > a base curve of +3.25. I'm not sure whether the discrepancy between > +3.25 and +4.00 for the old glasses is significant. Well, I drove out to pick up the corrected glasses today. It's certainly thicker and heavier on the higher-power left side. Seems to want to weigh down on the face more on the left side. The combination of higher base curve of 3 (from 1), lower index of 1.6 (from 1.67), and spherical lens shape seems to lessen the blurring at 45 degress to the side, but there is still some. The lab guy referred to this as "obliquely induced astigmatism", or "induced oblique astigmatism". He says it's unavoidable, and suggested I google it. I can't seem to find much in the way of such an effect for glasses, though it seems to seems be a common thing problem for the eyeball itself, as well as a common side effect of eye surgery. The lab guy's conclusion was that a higher base curve lessens the off-center blurring because the lens curves around the eye (like a panoramic). However, it yields a thicker lens. So functionally, a base curve of 5 would be ideal, but cosmetically the pits. They chose to compromise at 3, same as my old glasses. I wonder if the extra thickness of higher base curve (say, 5) could be offset by using the higher refractive index 1.67. Granted, the higher index is suppose to worsen the off-axis blur, too, so the net gain in terms of both blur reduction and lens thinness might very well be close to zero. Could anyone comment on this, even if it's just speculation? A friend of mine is convinced that the is something wrong when I explained the blurring. She has glass prescription of -4.75 (same as my old ones, before moving to this new -5.25) and says there is no blurring at all, in any of the field of view within the frame of the glasses. She is also sure of the same for some of her friends (though it's difficult to imagine people actually talking about such details in normal daily conversation). According to her, the switch to -5.25 is too minor to account for this off-axis blurring. I think my optician's lab guy also hinted that the increase wasn't that much. When I picked up my glasses today, however, I asked my optician why I didn't experience such a degree of off-access blurring with my old glasses, which has the same base curve, index, and spherical shape. She attributed it to the big change in the prescription. I wasn't sure how to interpret this, given the opinions that the change was not that big. Is the change from -4.75 to -5.25 "big"? I don't want to be making a fuss if the optician is right. I'm trying to be sensitive to the fact that it is a small-ish business and probably not as able to absorb the cost of catering to ill-founded complaints. If the problem is fundamental limitation in technology rather than design, it would be a waste of everyone's time (mines and theirs) to continually try different things (it takes a chunk of time to drive out there). I printed out Ian's details about fit of lens inside frame, and physical deformation of the lens by the frame. She's pretty adamant that they avoid that. I also showed her Robert's ideas on proper fit, and "abbe". She politely assured me that those are the things that she is trained in. In fact, the move from n=1.67 to 1.6 already implements one of the suggestions, though I'm sure it's not Seiko's n=1.6 aspheric lens -- this 2nd attempt at making the lens included moving to a spherical lens to replicate conditions of my old glasses as much as possible. However, since 3 parameters were changed at once, I'm not entirely sure which actually contribute to the improvement, and which may limit the improvement. I suspect that the higher base curve helps, for the reasons above (sure sounds logical). As well, I suspect that the lower index helps, since this seems to be the common opinion among those who work in this area. However, I wonder about the move to a spherical lens. Isn't the aspherical design suppose to make the clarity of sight more uniform throughout the entire field of view, when looking straight ahead as well as looking off-axis? I also looked up abbe. It seems to be another way to characterize the variation of refractive index with color, so now I see why Robert mentioned this in the context of "transverse chromatic aberration". I should have asked if it was a low-abbe material. Perhaps I'll call back to ask. Thanks for your suggestions and information. If you have any further comments on the thoughts I've written here, I would certainly welcome them. Fred >Well, I drove out to pick up the corrected glasses today. It's certainly >thicker and heavier on the higher-power left side. Seems to want to weigh [quoted text clipped - 3 lines] >side, but there is still some. The lab guy referred to this as "obliquely >induced astigmatism", or "induced oblique astigmatism". Oblique astigmatism can be minimized by using the correct "best form" base curve and by positioning the lenses properly in front of the eyes. For any given Rx and lens design there is one correct base curve. The reason for the off-axis blur is a combination of the low abbe value of 1.67 index and/or a poor fit. >He says it's >unavoidable, and suggested I google it. I can't seem to find much in the [quoted text clipped - 5 lines] >curve of 5 would be ideal, but cosmetically the pits. They >chose to compromise at 3, same as my old glasses. If you use the wrong base curve there will be an increase in oblique astigmatism and spherical power error. If these lenses were in "finished form", meaning off the shelve and ready to be cut and edged into the frame, the base curve will be correct. If the lens was surfaced or ground by the lab then there is a chance the wrong curve was used. The center thickness is about 1.0mm with finished lenses and usually no less than 1.3mm if surfaced, and can be (and usually are) much thicker than that depending on good the lab is. Have the optician check the center thickness also. >I wonder if the extra thickness of higher base curve (say, 5) could be >offset by using the higher refractive index 1.67. Granted, the higher >index is suppose to worsen the off-axis blur, too, so the net gain in >terms of both blur reduction and lens thinness might very well be close >to zero. Could anyone comment on this, even if it's just speculation? If you use a different base curve than called for by the manufacturer of the lens then there will be an increase in off-axis blur. >A friend of mine is convinced that the is something wrong when I explained >the blurring. She has glass prescription of -4.75 (same as my old ones, [quoted text clipped - 35 lines] >the clarity of sight more uniform throughout the entire field of view, when >looking straight ahead as well as looking off-axis? Aspheric designs allows the use of flatter base curves without increasing oblique astigmatism. Flatter front curves reduce the weight and thickness of the lenses. No reason not to use an aspheric design just because your old lenses were not aspheric, unless the optician is inexperienced. Aspheric designs require more accuracy when positioned in front of the eyes. The off-axis blur is due to the low abbe and/or poor frame/lens fit. >I also looked up abbe. It seems to be another way to characterize the >variation of refractive index with color, so now I see why Robert mentioned >this in the context of "transverse chromatic aberration". I should have >asked if it was a low-abbe material. Perhaps I'll call back to ask. As I mentioned in my original post, the abbe value of 1.60 index varies from 36 to 42. The newer and better lenses are 42. >Thanks for your suggestions and information. If you have any further >comments on the thoughts I've written here, I would certainly welcome them. > >Fred Hope this helps Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > Hope this helps > [quoted text clipped - 3 lines] > Roberts Optical > robopt@execpc.com Yes, it does. The lab guy also phoned me and corrected the terminology I should google for. It should be "oblique incidence astigmatism", not "oblique[ly] induced astigmatism". That turns up some useful information. Thanks for your additional information about the sensitivity to positioning, abbe & tranverse chromatic aberration, sensitivity to base curve, and background on pre-prepared lenses. I have to figure out what I am going to do. I haven't been able to wear these lenses for extended periods (can be disorienting). As well, the left lens is heavier with the spherical lower index material, and tends to sag. That doesn't help it's positioning, I suspect. Even properly positioned, I'm finding that the left vision on-axis is blurrier than the right eye. Not by much, but noticable. I'm hesitant to bug the optmetrist about checking the accuracy of the my eyes. I know that the lens matches the prescription accurately, because I had it checked. Thanks again for your additional details. Fred >> Hope this helps >> [quoted text clipped - 18 lines] >suspect. Even properly positioned, I'm finding that >the left vision on-axis is blurrier than the right eye. It sounds like there is a problem with the left lens. I thought you wrote that the Rx was -5.75 sph, and either the same or very close in each eye. The left lens should be about the same thickness than the right lens. In addition, switching from 1.67 to 1.60 will increase the thickness about an 9% , with the change from spherical to aspheric adding another 10% or so. In most small frames the lower index lens will be about 1mm to at most 1.5mm thicker. >Not by much, but noticable. I'm hesitant to bug the >optmetrist about checking the accuracy of the my eyes. >I know that the lens matches the prescription accurately, >because I had it checked. I think you should let the doc see the glasses if this hasn't been done yet. Perhaps the optometrist will give the glasses a more thorough inspection than the others have provided. >Thanks again for your additional details. > >Fred Good luck Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical robopt@execpc.com ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman |
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