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Medical Forum / General / Vision / January 2009Different types of plastic 1.6?
I've got a high negative prescription so 1.5 index lenses are a little bulky. The trouble is I seem to be very sensitive to dispersion (chromatic aberration) in other materials. Several years ago (2002 IIRC) I got 1.6 plastic lenses from Specsavers (UK) and I was comfortable with them; I could see a little colour fringing at the edges but I felt I could see properly with them. Ever since then, whenever I've tried to go above 1.5 I can't tolerate the dispersion. It encroaches into my main field of vision, making it difficult to read, or to even do things like make a cup of tea because you need to look down at the counter, so I either have to look through the bottom of the lenses or hunch my back. No matter how long I wear them for the general feeling of not being able to see properly and getting headaches and a stiff neck because I have to keep moving my head instead of my eyes never goes away. In some cases it turned out that they'd used polycarbonate lenses, the very worst material for this effect, but the opticians I go to never seem to admit that they've ever heard of this problem. I thought seeing as Specsavers managed to make me a pair of glasses with 1.6 lenses that I could see through before, they could probably do it again, but I've got the problem again with my new glasses. Is the most likely explanation that Specsavers have started using a different material in the meantime? I think they only started associating themselves with the Pentax branding in the years since I got my last pair from them. According to <http://en.wikipedia.org/wiki/Corrective_lenses> there are at least two different types of 1.6 plastic (MR-8 having a higher Abbe value than MR-6) and polycarbonate has an index of 1.59 so it's quite likely that this would be sold as "1.6", although I specifically stated I didn't want that. Does anyone here know exactly what sort of lenses Specsavers use and used to use?  SignatureTH * http://www.realh.co.uk >I've got a high negative prescription so 1.5 index lenses are a little >bulky. The trouble is I seem to be very sensitive to dispersion [quoted text clipped - 11 lines] >getting headaches and a stiff neck because I have to keep moving my head >instead of my eyes never goes away. If you got along well with the 1.60 material in 2002, I would think you should be able to do so again, as long as there hasn't been a substantial increase in lens power, and the lens is positioned correctly. >In some cases it turned out that they'd used polycarbonate lenses, the >very worst material for this effect, but the opticians I go to never >seem to admit that they've ever heard of this problem. Lack of experience and/or training. >I thought seeing as Specsavers managed to make me a pair of glasses with >1.6 lenses that I could see through before, they could probably do it [quoted text clipped - 9 lines] >want that. Does anyone here know exactly what sort of lenses Specsavers >use and used to use? They should use the lens that you want- the lens that would be best for you as an individual, within your eyeglass budget. If they don't have what you need then go elsewhere. It's usually best to avoid the discounters if you're looking for precision and good service. Other lens materials that should be on the table are Finalite from Zeiss/Sola (1.60 but lighter than MR-6 and 8) and 1.70 from Hoya. Trivex might be too thick if your Rx is -8.00 or more. Consider more advanced designs from Shamir (Autograph 2) and Hoya (bi-aspheric Nulux EP). Hope this helps Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman [OK with old 1.6 lenses, too much dispersion with new ones] > They should use the lens that you want- the lens that would be best > for you as an individual, within your eyeglass budget. If they don't > have what you need then go elsewhere. It's usually best to avoid the > discounters if you're looking for precision and good service. I said to the dispenser right from the beginning that I had this problem and he seemed to be quite aware of what I meant, but reckoned that I'd be OK with their 1.6 now if I had been before :-(. > Other lens materials that should be on the table are Finalite from > Zeiss/Sola (1.60 but lighter than MR-6 and 8) and 1.70 from Hoya. > Trivex might be too thick if your Rx is -8.00 or more. Consider more > advanced designs from Shamir (Autograph 2) and Hoya (bi-aspheric Nulux > EP). Most of those sound a bit expensive. My prescription is -6.25sph -0.75cyl, -6.75sph -0.50cyl. I'm currently wearing 1.5 so Trivex could be OK, although the new frames are thinner. 1.70 looks like the Abbe would be too low, it's not much better than polycarbonate. I don't think I've kept my prescription from 2002, but the changes have been very minor in the last few years (since 2006 the only change is the axis by about 5deg). It's annoying that I was able to have a truly useful reduction in thickness for moderate extra cost back then which doesn't seem possible now, but I don't know why. I suspect there's more profit in materials with lower Abbe values. I suppose my next step is to ask exactly what type the new lenses are, and preferably what type the old ones were too, and if they can't tell me anything better than "Pentax" and "1.6" it'll be obvious they can't provide me with the level of service I need and I'll ask for a refund. Thanks. SignatureTH * http://www.realh.co.uk >[OK with old 1.6 lenses, too much dispersion with new ones] > [quoted text clipped - 6 lines] >and he seemed to be quite aware of what I meant, but reckoned that I'd >be OK with their 1.6 now if I had been before :-(. Right. But something has changed- either you're more sensitive to these aberrations (unlikely), a significant change in Rx (seems unlikely) the lenses are fit incorrectly (common), and/or they subbed Poly for 1.60 index. >> Other lens materials that should be on the table are Finalite from >> Zeiss/Sola (1.60 but lighter than MR-6 and 8) and 1.70 from Hoya. [quoted text clipped - 3 lines] > >Most of those sound a bit expensive. They are expensive. >My prescription is -6.25sph >-0.75cyl, -6.75sph -0.50cyl. Not so strong at all. You would probably see little benefit from the above lens technology (diminishing returns). >I'm currently wearing 1.5 so Trivex could >be OK, although the new frames are thinner. Frame thickness is generally not a concern with minus powers. Trivex would be less than 10% thinner (assuming both have center thickness around 1.2mm- can't get cr39 that thin in the US). Substantially lighter though, and more impact resistant. >1.70 looks like the Abbe >would be too low, it's not much better than polycarbonate. Most find it satisfactory even at powers in the mid-teens, although if you're extremely sensitive to chroma I'd play it safe and avoid it. >I don't think I've kept my prescription from 2002, but the changes have >been very minor in the last few years (since 2006 the only change is the >axis by about 5deg). It's annoying that I was able to have a truly >useful reduction in thickness for moderate extra cost back then which >doesn't seem possible now, but I don't know why. A good optician will sort it out. It looks like Rx changes can be ruled out. >I suspect there's more >profit in materials with lower Abbe values. In general, as the index of refraction goes up the Abbe value goes down. Has more to do with the physics of light than profit and loss. >I suppose my next step is to ask exactly what type the new lenses are, >and preferably what type the old ones were too, and if they can't tell >me anything better than "Pentax" and "1.6" it'll be obvious they can't >provide me with the level of service I need and I'll ask for a refund. > >Thanks. Good luck, and don't concentrate solely on material and lens design- aberrations and power error can easily be induced by other factors. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > >[OK with old 1.6 lenses, too much dispersion with new ones] > > [quoted text clipped - 10 lines] > aberrations (unlikely), a significant change in Rx (seems unlikely) the lenses > are fit incorrectly (common), and/or they subbed Poly for 1.60 index. We can rule out the first because I've still got the old glasses. I could have just stuck with them I suppose, but I was fed up with the frame. And is it my imagination, or do the coatings deteriorate in some way and get harder to clean after time? [Snip] > >I don't think I've kept my prescription from 2002, but the changes have > >been very minor in the last few years (since 2006 the only change is the [quoted text clipped - 3 lines] > > A good optician will sort it out. It looks like Rx changes can be ruled out. Finding a good one here seems to be the difficulty :-(. > >I suspect there's more > >profit in materials with lower Abbe values. > > In general, as the index of refraction goes up the Abbe value goes down. Has > more to do with the physics of light than profit and loss. Yes, but some materials are better than others at similar indexes. At opticians in the UK I always feel like they're hard-selling polycarbonate (usually dressed up in an impressive-sounding brand name). The raw material can't be expensive because they use it for things like roofs, while if there's an additional difficulty in machining it, surely they've all got the right/improved tools etc by now and that cost is largely amortized, but they can still charge the customer more for "thinner, stronger, lighter". SignatureTH * http://www.realh.co.uk >> Right. But something has changed- either you're more sensitive to these >> aberrations (unlikely), a significant change in Rx (seems unlikely) the lenses [quoted text clipped - 4 lines] >frame. And is it my imagination, or do the coatings deteriorate in some >way and get harder to clean after time? Yep. Given enough time they'll fail completely. High quality coatings receiving proper care should last 4+ years. >> A good optician will sort it out. It looks like Rx changes can be ruled out. > >Finding a good one here seems to be the difficulty :-(. The independents and ophthalmologists attract the most skilled and experienced opticians. Avoid the big boxes. >> >I suspect there's more >> >profit in materials with lower Abbe values. [quoted text clipped - 3 lines] > >Yes, but some materials are better than others at similar indexes. True. But better how? Poly is about the same index as 1.60 but has superior impact resistance (especially when AR coated) and is less expensive. Poly provides good value in the lower powers in a non-chemical environment. >opticians in the UK I always feel like they're hard-selling >polycarbonate (usually dressed up in an impressive-sounding brand name). [quoted text clipped - 3 lines] >largely amortized, but they can still charge the customer more for >"thinner, stronger, lighter". Poly (always hard coated) cost me more than hard coated cr39. For some, it's the best choice. You can protect yourself, somewhat, from opticians who do not have your "best interests at heart" by becoming an informed consumer. From what you've said so far, I'd probably narrow the lens choice to properly fit aspheric Spectralite or Trivex, although crown glass, cr39, and 1.60 should be on the table. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman > >My prescription is -6.25sph -0.75cyl, -6.75sph -0.50cyl. Axis 10, 37. > Good luck, and don't concentrate solely on material and lens design- > aberrations and power error can easily be induced by other factors. I found my old prescription from 2003 which must be the one the old 1.6 glasses were made from. It's -6.00,-0.75,10;-6.25,-0.5,40. So quite a small difference. I was wondering it it was possible the astigmatism correction could exacerbate the distortion, but with such a small change in prescription that doesn't seem a likely explanation at all. SignatureTH * http://www.realh.co.uk |
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