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Medical Forum / General / Vision / April 20071.7 vs 1.53 refractive index in glasses ??
Hi there, I finally decided to get new glasses (after having the previous pair for around 10 years). The modern frames are small and narrow which is good for people who have long-sightedness but no good for the people with miopia like myself. Finally, I was able to choose one. I tried the plastic lens (polycarb ?) , and did not like it because the image at the edge of the lens was annoyingly less sharp than in my glass lenses (refractive index 1.53 ?). I think that the higher-index lenses have the higher optical aberrations (which is seen as a less sharp image especially at the lens edge) because the light has to bend to a higher degree. I went to another optometrist, and told him I would accept only glass lenses (he nevertheless tried to sell me plastic). He suggested me to get the high-index glass of the 1.7 refractive index instead of 1.53. Is this going to get me more aberrations ? What are the people's experiences about the 1.7 glass compared to 1.53 ? (I value the quality of the image above of everything. I do not care about the weight. I do not want to get anti-reflective coating because it will scratch. My lenses are -4 dioptre. The lens frame is 50 mm across.) Thanks. > Hi there, > [quoted text clipped - 17 lines] > it will scratch. My lenses are -4 dioptre. The lens frame is 50 mm > across.) Thanks. In general, the higher the refractive index the lower the aberations. Please visit my web site at www.richardfisher.com >In general, the higher the refractive index the lower the aberations. I believe you meant to write the opposite- higher index of refraction ophthalmic lens materials usually have increased chromatic aberration/lower Abbe value. For instance, the Abbe value of crown glass (1.523) is about 59, 1.60 is about 42, and 1.67 is about 33. Regards, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > >In general, the higher the refractive index the lower the aberations. > [quoted text clipped - 13 lines] > "Science is a way of trying not to fool yourself." > - Richard Feynman The higher the refractive index the lower the aberations. The only exception is chromatic aberration. Please visit my web site at www.richardfisher.com >> >In general, the higher the refractive index the lower the aberations. >> [quoted text clipped - 18 lines] > >Please visit my web site at www.richardfisher.com Richard, The monochromatic aberrations that ophthalmic lens designers are most concerned with are oblique astigmatism and power error. Spherical aberration (below about +12.00D) and coma do not seem to be a concern because of our small pupil size. Distortion can not be eliminated by changes in surface design (base curve and asphericity). So, I ran a series of lens powers through a nifty "Optical Analysis" program written by Darryl Meister, the technical marketing manager for Carl Zeiss Vision. Using low index cr39 plastic, and lens powers from -15.00D to +8.00D, I was able to get very low numbers (less than .25D) for oblique astigmatism, RMS power error, and mean power error by manipulating the spherical base curve (plano base curve for the -15.00D and +12 base curve for the +8.00D). When switching to very high index materials (Zeiss 1.9 glass), the best case power error was greater (about +.50D in high minus). I was only able to get better numbers (at or below .25D) with moderate to high plus powers (+5.00 to +8.00) by using very steep (+15D to +19D) base curves, curves that are both impractical and generally unavailable for ophthalmic use. For thin ophthalmic lenses, the aberrations mentioned above seem to be slightly greater as the index of refraction increases. However, I'm just a dispensing optician who does not fabricate lenses, and have at best, a very modest math and physics background. I'll submit this to another forum where there are some savvy lens designers who do ray tracing, and don't shy away from advanced trigonometry like I do. :) Regards, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman >The higher the refractive index the lower the aberations. The only >exception is chromatic aberration. > > You may or may not be correct on that; I'd be interested in knowing where you get your data. However, even if it's true that things like spherical aberration and coma vary inversely with refractive index, I'd have to hazard a guess that chromatic aberration must be the most bothersome of all aberrations in eyeglasses... > The higher the refractive index the lower the aberations. The only > exception is chromatic aberration. Dear unhelpful person: Chromatic aberration is the most insidious kind of aberration in eyeglasses since it makes images appear as a blurry (under normal lighting) as the different colors in the light spectrum focus in different positions (even for black text on white paper). Other kinds of aberrations such a pincushion distortion, barrel distortion, etc, etc may cause straight lines to appear slightly curved, but these distortions do not generally make the image blurry. > > The higher the refractive index the lower the aberations. The only > > exception is chromatic aberration. [quoted text clipped - 9 lines] > etc, etc may cause straight lines to appear slightly curved, but these > distortions do not generally make the image blurry. Ever heard of spherical aberration, coma, astigmatism and higher order monochromatic aberrations? > Ever heard of spherical aberration, coma, astigmatism and higher order > monochromatic aberrations? Yes, I have heard of most of them. None of them are nearly as important to eyeglasses as is chromatic aberration. Chromatic aberration blurs the image (not just distorts it) and the aberration is generally higher (abbe value is lower) as the lens index of refraction increases. There are exceptions, especially Polycarbonate lens material (1.59 index) which has an abbe value of 30, even though there are many higher index lens materials (1.67 etc) that have an abbe value of 32 or higher (the higher the abbe value, the lower the chromatic aberration). Almost every lens manufacturer will specify the abbe values of their materials to help ECP and consumers make the proper choices and trade-offs for the patient. I have never seen a lens manufacturer specify values for the other aberrations you have mentioned. That does not mean the other aberrations you mentioned don't exist, but they are not significant enough to worry about in modern lens designs for eyeglasses. > > Ever heard of spherical aberration, coma, astigmatism and higher order > > monochromatic aberrations? [quoted text clipped - 5 lines] > aberration is generally higher (abbe value is lower) as the lens index of > refraction increases. Spherical aberration and coma will also blur the image. Here the situation actually improves with higher index, as less curvature is required for a given lens strength. Mark >The higher the refractive index the lower the aberations. The only >exception is chromatic aberration. Richard, I asked Mr. Meister about this at another ophthalmic optics forum- www.optiboard.com He did not believe (except for chromatic aberration) that this would be true for spectacle lenses because the ideal image plane of the eye is curved, resulting in significant curvature of field, as opposed to an optical system with a flat image plane, where curvature of field may be reduced by increasing the refractive index. It appears that for a spectacle lens, there is no optical advantage in using higher refractive index materials, and that there are disadvantages related to increased chromatic aberration as you have stated. Regards, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > Hi there, > [quoted text clipped - 17 lines] > it will scratch. My lenses are -4 dioptre. The lens frame is 50 mm > across.) Thanks. Please post your Rx so that a proper recommendation of a lens can be made. Polycarb is 1.59, not 1.53. If it is 1.53, then it may be Trivex or some other material. 1.70 should only be used for a very strong Rx to help reduce the thickness of the lens. As already noted by another post, the higher the index, the worst the optical qualities of the lens, except for polycarb which has the worst optical qualities of any commonly dispensed lens. > > Hi there, > [quoted text clipped - 27 lines] > worst the optical qualities of the lens, except for polycarb which has the > worst optical qualities of any commonly dispensed lens. Here is the article from Wikipedia. It says that lenses from the materials with the higher refractive index have the higher chromatic aberrations. The higher chromatic aberrations are perceived by people as a higher "blurryness". Perhaps this is what I was witnessing. The materials with the higher dispesion are said to have the higher Abbe number (calculated according to the formula). The higher the number the less is the dispesion, therefore, the better is the optical quality. In the link below, it is stated that the Abbe number of plastic (CR 39) is 59.3, which is on par with the low (1.53)- refractive index crown glass (58.5). It is much higher than the Abbe number of a high-refractive index glass of a 1.7 refractive index, namely 36 for Hoya glass. This should mean that the polycarb should have a very good image quality from the point of view of chromatic aberration -- the same as that of glass, in fact. But you and others say that polycarb is bad. How come ? http://en.wikipedia.org/wiki/Corrective_lens#Plastic_.28CR-39.E2.84.A2.29 >Here is the article from Wikipedia. It says that lenses from the >materials with the higher refractive index have the higher chromatic >aberrations. It says the higher the IR the lower the Abbe value. The lower the Abbe value the greater the dispersion hence greater transverse chromatic aberration (TCA). This is the chromatic aberration created by the prismatic effect at the periphery of a lens when looking off axis. The formula for TCA is cF/V, where c is the distance from the OC (optical center) in centimeters, F is lens power and V is the Abbe value. According to Torgersen, acuity is not affected until TCA reaches 0.16 prism diopters and then it drops by one line on the Snellen chart. >The higher chromatic aberrations are perceived by people >as a higher "blurryness". Perhaps this is what I was witnessing. The [quoted text clipped - 6 lines] >number of a high-refractive index glass of a 1.7 refractive index, >namely 36 for Hoya glass. >This should mean that the polycarb should have a very good image >quality from the point of view of chromatic aberration -- the same as >that of glass, in fact. But you and others say that polycarb is bad. >How come ? See the above. >http://en.wikipedia.org/wiki/Corrective_lens#Plastic_.28CR-39.E2.84.A2.29 Lot's of good information although there are a few errors and misinformation. For instance, we do not use bi-convex lenses to correct presbyopia due to unacceptable levels of oblique astigmatism and power error. With rare exception, ophthalmic lenses use best form meniscus shaped lenses. Back to your original question- I still haven't heard a reason why you should change your lens material from crown glass, although many optical companies (primarily the chains) will not supply glass lenses. Most use polycarbonate due to profitability and reduce processing time. Hope this helps, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > >Here is the article from Wikipedia. It says that lenses from the > >materials with the higher refractive index have the higher chromatic [quoted text clipped - 36 lines] > (primarily the chains) will not supply glass lenses. Most use polycarbonate due > to profitability and reduce processing time. OK, here is the full story. I went to the optometrist for new spectacles (in Oz). He offered me the polycarb lenses, and I said I did not want them, because they produced too much blur in the peripheral vision, and thus I wanted glass. He recommended me the glass of a 1.7 refractive index because it was lighter than the standard glass (1.53 crown). I did not know why the 1.7 glass should be optically worse or how much it would be worse, so I reluctantly agreed. When I came home, I did research on the Internet (and found the above- mentioned article on Wiki). It said that the 1.7 glass had the dispersion as bad as the one of the polycarb. The 1.53 crown glass had much lesser dispersion (its Abbe number was ~60 vs. ~30 of the polycarb/1.7 glass). I became unhappy about my future glasses :-( . When I came to pick up the ready glasses, I saw that my new glasses produced as blurry image on periphery as the polycarb-fitted glasses did. When I moved my gaze 1 cm away from the lens centre towards the periphery (what, 20 degrees ?), the image became very blurry. It was totally inacceptable to me. I said to the optometrist about it. I showed him the print-out from Wiki with the data on Abbe numbers of 1.53 and 1.7 glass and of polycarb. The optometrist told me that he wanted to keep me happy, and I did not have to buy the glasses. I asked him to fit the ordinary crown glass lenses into the frame (to replace the 1.7 glass), and said I would pay for the old and new lenses. The old chap said he already offered the best spectacles for me, and he would not be doing anything more for me. He suggested I take my prescription and go to another optometrist. I could not believe my ears. I decided that the old guy was not in the business for money any more (he was practising in this prime downtown place for 50 years); he was doing the business just for job satisfaction. I decided that I wanted to try these low-Abbe number lenses anyway, so I bought the glasses. As an afterthought, the optometrist told me that I could bring the glasses for refund within 2 weeks if I find them unsuitable. I now have these glasses. I calculated the transverse chromatic abberation (TCA) according to your formula above, and actually found that the Torgersen criterion was correct (the TCA was around 0.33 D and the acuity actually dropped by about 2 lines). Interestingly, my old glasses with the 1.53 crown glass lenses also had an acuity drop at the same peripheral angle as the new glasses, however the drop was not as much (maybe 0.5-1 line ?). Heck, today I was watching for the babes walking past me, and I had to turn my head to see them better (thus, it became evident to my companion that I was actually watching them) (with my old glasses, I did not have to turn my head and still saw them clearly at the periphery of the lens !). Also, the 1.7 glass lenses give a purple or green reflection (dependent on the viewing angle). This tells me that the lenses have an anti-reflection (or any other) coating. The optometrist told me that the lenses had no coating just like I requested. Is there a coating on my lenses ? Thanks for your attention. >> >The higher chromatic aberrations are perceived by people >> >as a higher "blurryness". That's a misconception. Chromatic aberration separates colors so that sharp black-white edges have a red or blue "fringe". It's also common in cheap binoculars and telescopes. It's undetectable in low-power polycarb. >> >This should mean that the polycarb should have a very good image >> >quality from the point of view of chromatic aberration -- the same as >> >that of glass, in fact. But you and others say that polycarb is bad. >> >How come ? Sometimes it's because they sell a competing product. > When I came to pick up the ready glasses, I saw that my new glasses > produced as blurry image on periphery as the polycarb-fitted glasses > did. When I moved my gaze 1 cm away from the lens centre towards the > periphery (what, 20 degrees ?), the image became very blurry. That doesn't sound like chromatic dispersion, unless there were color fringes. > Also, the 1.7 glass lenses give a purple or green reflection > (dependent on the viewing angle). This tells me that the lenses have > an anti-reflection (or any other) coating. The optometrist told me > that the lenses had no coating just like I requested. Is there a > coating on my lenses ? Yes. AR coating has that property. -MT > >> >The higher chromatic aberrations are perceived by people > >> >as a higher "blurryness". [quoted text clipped - 25 lines] > > Yes. AR coating has that property. You remind me of the picture frame I once purchased with 'low reflectance glass' In fact, there was no glass in it at all. Effective, mind. > -MT >>> >The higher chromatic aberrations are perceived by people >>> >as a higher "blurryness". > > That's a misconception. Chromatic aberration separates colors so that > sharp black-white edges have a red or blue "fringe". It's also common in > cheap binoculars and telescopes. It's undetectable in low-power polycarb. You are both wrong. My polycarb lenses produce an annoying chromatic aberration that is not present in my plastic (I can't quite think of the name right now, the ordinary stuff) do not. I just got a pair of plastic glasses and polycarb safety glasses; when driving the difference is quite obvious on the tops of trees. However, they are not particularly blurry, and you get used to it. My old lenses were polycarb and I stopped noticing the colors after a while. I only notice it now with my safety glasses because I don't often wear them outside. On 3/24/07 3:55 AM, in article 1174733747.758668.305930@e65g2000hsc.googlegroups.com, "carrera d'olbani" <dolbani@yahoo.com> wrote: > OK, here is the full story. I went to the optometrist for new > spectacles (in Oz). He offered me the polycarb lenses, and I said I [quoted text clipped - 4 lines] > be optically worse or how much it would be worse, so I reluctantly > agreed. It would be good to get a review of modern glasses and the links between density and index of refraction. In the old days, light (low density) glass was crown glass with a low index of refraction and dense glass was flint glass with a relatively high index of refraction. High density material had a high density of bound electrons (complicated atoms) that contributed to index of refraction. Because the power of a lens was proportional to n-1, it was never clear to me that you could get lower weight optics (spectacles) by shifting to higher index material. That is, there is no guarantee that thinner also was lighter. These days, with fancy materials such as rare earth glasses and who knows what kind of plastics, the relationship between density and index can be more obscure. Can anyone supply a link that will clarify these relationships? Bill -- Fermez le Bush--about two years to go. >OK, here is the full story. Posting your Rx would be helpful. >I went to the optometrist for new >spectacles (in Oz). He offered me the polycarb lenses, and I said I [quoted text clipped - 15 lines] >did. When I moved my gaze 1 cm away from the lens centre towards the >periphery (what, 20 degrees ?), About 40 degrees of ocular rotation. >the image became very blurry. It was >totally inacceptable to me. I said to the optometrist about it. I [quoted text clipped - 21 lines] >that the Torgersen criterion was correct (the TCA was around 0.33 D >and the acuity actually dropped by about 2 lines). So your Rx is about -10.00 assuming 1cm off-axis and a material Abbe value = 30. >Interestingly, my >old glasses with the 1.53 crown glass lenses also had an acuity drop >at the same peripheral angle as the new glasses, however the drop was >not as much (maybe 0.5-1 line ?). There are other ways to blur the vision off-axis (on-axis as well). If the base curve (front curve) is not "best form" then there will be blurring due to oblique astigmatism and power error. For example, a -10.00 crown glass lens with a back vertex distance of 13mm should have a base curve of +1.00, which results in .34DC of astigmatism and +.25 power error. If +2.00 base is used then those numbers become .30DC and +.75 respectively. Using a plano base gives 1.12 of astigmatism and +.75 power error. >Heck, today I was watching for the >babes walking past me, and I had to turn my head to see them better >(thus, it became evident to my companion that I was actually watching >them) (with my old glasses, I did not have to turn my head and still >saw them clearly at the periphery of the lens !). I'll have to add that one to my repertoire. >Also, the 1.7 glass lenses give a purple or green reflection >(dependent on the viewing angle). This tells me that the lenses have >an anti-reflection (or any other) coating. The optometrist told me >that the lenses had no coating just like I requested. Is there a >coating on my lenses ? You shouldn't see the "reflex" color looking through the lens, unless it's due to chroma. If you remove the glasses and hold them at different angles then you will see the reflex color if they are coated. I can't imagine why you wouldn't want coated optics. >Thanks for your attention. Your welcome. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > Posting your Rx would be helpful. I posted it in the very first message. It is -4 dioptre. > I can't imagine why you wouldn't want coated optics. My previous glasses lasted me for 9 years. I wanted my new glasses to last for about the same. Sometimes I work in dusty conditions. When I wipe the glasses, the coarse dust can scratch the AR coating off. In fact, I just noticed a tiny scratch on the AR coating. I guess I could bring the glasses back to the optometrist for a refund, but... I doubt that I could get anything better anywhere else. It is all the same. I am just recalling the reason why agreed to order the glasses with the high RI, and then buy them when it came out that the glasses got AR coating (contrary to what I ordered) was that I wanted to try something "modern" and "high-tech". Oh well, you gotta pay for the "toys". >> Posting your Rx would be helpful. > >I posted it in the very first message. It is -4 dioptre. I thought I read the whole post! Sorry about that. >> I can't imagine why you wouldn't want coated optics. > >My previous glasses lasted me for 9 years. I wanted my new glasses to >last for about the same. Sometimes I work in dusty conditions. When I >wipe the glasses, the coarse dust can scratch the AR coating off. In >fact, I just noticed a tiny scratch on the AR coating. If you can't rinse the lenses with water then uncoated is going to be more practical. >I guess I could bring the glasses back to the optometrist for a >refund, but... I doubt that I could get anything better anywhere else. [quoted text clipped - 3 lines] >wanted to try something "modern" and "high-tech". Oh well, you gotta >pay for the "toys". Some lens companies will only produce lenses that are AR coated- Hoya ID and Zeiss Individual are two examples. However, if the optician recommended a different product than what you wanted, they should exchange it for uncoated crown glass if you are dissatisfied with the performance of higher index coated lenses. Regards, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > .....although many optical companies > (primarily the chains) will not supply glass lenses. Most usepolycarbonatedue [quoted text clipped - 10 lines] > "Science is a way of trying not to fool yourself." > - Richard Feynman Robert, I have been reading with great interest the information you & others here have provided regarding the differences in lens choices for progressive lenses. Being in my mid-40's, I am now in need of getting my first pair of progressive lenses filled. My Rx is currently a: -50/ PL with a +1.75 for reading. Working as a waiter during lunch in a fine dining establishment, it would be far too inconvenient for me to resort to putting on/off my non-Rx readers every time I'm about to write an order while standing at a guests table! Hence, the need for progressive lenses. Yet, after getting quotes with 3 different national optical chains today I can only see NOW how each one was 'pushing' so strongly for me to go with polycarbonate lenses (Panoptic-short by Duralite???) over plastic C-39, never mind the dramatic cost difference! Being this is my first time trying progressive lenses and my Rx, Do I need the polycarbonate lenses for safety reasons of impact resistance or would C-39 do as well? Also, if I go with the plastic lenses should I opt for either the UV, Scratch, or AR coatings? Thanks in advance for any help you and the others can lend.... >> .....although many optical companies >> (primarily the chains) will not supply glass lenses. Most usepolycarbonatedue [quoted text clipped - 18 lines] >my first pair of progressive lenses filled. My Rx is currently a: -50/ >PL with a +1.75 for reading. Ask about lenses that have long corridors and reduced unwanted surface astigmatism. Examples would be Zeiss's Individual and Essilor's Definity. If you're an avid reader then other designs might be more appropriate. The easiest lens to adapt to would probably be Hoya's ID, but it's very expensive. However, even the best lens will perform poorly when positioned improperly in front of the eyes. For that reason I would choose a presbyopic optician who has a great deal of experience fitting older clients. An optician located in an ophthalmologist's office or one who works primarily with MDs deserves consideration. Avoid trainees, clerks, stylists, and younger opticians in general. >Working as a waiter during lunch in a fine dining establishment, it >would be far too inconvenient for me to resort to putting on/off my [quoted text clipped - 5 lines] >to go with polycarbonate lenses (Panoptic-short by Duralite???) over >plastic C-39, never mind the dramatic cost difference! A lighter weight lens (and frame) would be advantageous for a first time wearer of glasses. >Being this is my first time trying progressive lenses and my Rx, Do I >need the polycarbonate lenses for safety reasons of impact resistance >or would C-39 do as well? Also, if I go with the plastic lenses >should I opt for either the UV, Scratch, or AR coatings? Depends on the frame- drill mounts require lenses with greater tensile strength. Uv is probably not an issue unless you spend a lot of time outdoors and/or live at a higher altitude or lower latitude. Most progressives have scratch coatings and most if not all non-glass and cr39 lenses have UV inhibitors in the material. If you get AR (generally recommended) get the best- easy to clean and guaranteed against defects for two years. >Thanks in advance for any help you and the others can lend.... Your welcome. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman >This should mean that the polycarb should have a very good image >quality from the point of view of chromatic aberration -- the same as >that of glass, in fact. But you and others say that polycarb is bad. >How come ? > > You managed to find a glass that is just as BAD as polycarb. Stick with regular crown glass. high ABBE=good low ABBE=bad > Here is the article from Wikipedia. It says that lenses from the > materials with the higher refractive index have the higher chromatic [quoted text clipped - 13 lines] > that of glass, in fact. But you and others say that polycarb is bad. > How come ? Because polycarb (1.59 index) is a fundamentally different material than other "plastics" and has a completely different molecular structure. The one advantage of polycarb is high impact resistance and high tensile strength. It has an abbe value of 30, which is the lowest (worst) of any commonly dispensed lens material. Trivex has about the same impact resistance and tensile strength as polycarb, but is 1.53 index, and has a very good abbe value of about 42. >Hi there, > [quoted text clipped - 17 lines] >it will scratch. My lenses are -4 dioptre. The lens frame is 50 mm >across.) Thanks. If weight and safety are not an issue then stay with crown glass. Is thickness an issue? I would strongly recommend an AR coat. Clean with soap and water and wipe dry with a microfiber cloth (Luminex makes the best). If you work in a dusty environment without access to water, an AR coating is probably not going to be practical, although you could order one pair for work w/o AR and another pair with AR for outside of work. Hope this helps, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman >Hi there, > >I finally decided to get new glasses (after having the previous pair >for around 10 years). The modern frames are small and narrow which is >good for people who have long-sightedness but no good for the people >with miopia like myself. I disagree with that. Smaller frames are good for anyone with a moderate to high power Rx so long as the eyes are fairly well centered in the lenses. That's because the lenses are thinner and the amount of your visual space that is covered up by the edges is narrower, so you actually get more vision. > Finally, I was able to choose one. I tried >the plastic lens (polycarb ?) , and did not like it because the image [quoted text clipped - 3 lines] >image especially at the lens edge) because the light has to bend to a >higher degree. You are right, and polycarb is the worst of all. > I went to another optometrist, and told him I would >accept only glass lenses (he nevertheless tried to sell me plastic). [quoted text clipped - 3 lines] > > Anything is better than poly, and glass is the best. >(I value the quality of the image above of everything. I do not care >about the weight. I do not want to get anti-reflective coating because >it will scratch. My lenses are -4 dioptre. The lens frame is 50 mm >across.) Thanks. > Anti reflection on glass lenses is very very durable. If you take halfway good care of them, they will be fine, and the "quality of the image above of everything" is definitely better with the AR coating. > Hi there, > [quoted text clipped - 17 lines] > it will scratch. My lenses are -4 dioptre. The lens frame is 50 mm > across.) Thanks. a good 1.7 glass (abbe 40 such as "fit 40") is probably not going to cause any noticeable chromatic abberation. polycarb is really bad. Good 1.6 index plastic (MR-8 material) should be fine if you go the plastic route (for less weight and higher impact resistance than glass).
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