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Medical Forum / General / Vision / January 2007Thinner lenses
Please can you advise what strength of prescription that you would recommend upgrading from a standard plastic to thinner and lighter lenses? Thanks > Please can you advise what strength of prescription that you would > recommend > upgrading from a standard plastic to thinner and lighter lenses? Thanks Would it be possible for you to post your Rx? Sure, the big numbers are -225 and -275. > Sure, the big numbers are -225 and -275. Actually the big numbers are -2.25 and -2.75, but like many medical professionals, ODs like to be sloppy in their hand written Rx's. I am not sure why you did not mention the cylinder and axis measurements, but unless they are unusually high it probably does not matter. Is your Rx for single vision or progressives (no line bifocals with add power in reading area)? If they are progressives, what is the add power? How sensitive are you to thick edges for cosmetic reasons (although -2.75 is not very thick even with regular plastic 1.50)? How sensitive are you to cost? A higher index lens will be a little thinner, but may cost a lot more. Also, the higher the index, the lower the optical quality of the lens (measured as abbe value). But with a moderate Rx like yours, a higher abbe value is not quite as noticeable as would be the case with a more powerful Rx. But I would avoid polycarb, which has a very low abbe value. Sola Spectralite (Sola or AO lenses only) at 1.53 index are a bit thinner than regular plastic and the material is lighter. Trivex (Hoya Phoenix material) is 1.54 index and is very strong and excellent for drill mounted lenses. I don't think you would want to go past 1.60 with such a moderate Rx (even though many opticians want to sell 1.67 to everyone who walks in the store). > Sure, the big numbers are -225 and -275. It also depends on how big the frame you've chosen is, and what it's made of. A small frame made of plastic (=thicker frame edges) conceals edge thickness much better than a large frame made of metal (=thinner frame edges). So if you're buying a big and/or metal frame then it's more worthwhile paying for high-index (thin) lenses. And, it *also* depends on your PD (= distance between your eyes)... a person with a smaller PD (ie. close-set eyes) will get more benefit from a high-index lens, all else being equal. Having said all of that, I generally start to recommend high-index lenses at around -2.50 to -3.00 ... but there are no hard & fast rules about it. Dom Thanks Mark A & Dom The frames I like are metal so it may be worth it. I'm happy to pay for something that makes a difference to the cosmetic effect. Is it possible to find out what the difference between the regular and thinner lenses would measure? I've noted your comment about not going above 1.60, that's helpful. So for metal frames, you'd definately recommend. > Thanks Mark A & Dom > [quoted text clipped - 5 lines] > I've noted your comment about not going above 1.60, that's helpful. So for > metal frames, you'd definately recommend. For your Rx, I would not go over 1.60, but I am more concerned about optical quality than some people. There are some people who would get 1.67 for your Rx. As the lens index goes higher, the lens thickness is less for a given Rx, but chromatic aberration and distortion is higher. If you had a much stronger Rx, then your lenses edges would be a lot thicker and 1.67 would be justified. So it is really a just a trade-off between thickness, cost, and optical quality of the lens. If you supply the exact lens model you are thinking of (manufacturer and model) then it would be possible to determine the difference in edge thickness of the various materials. Getting an aspheric design may also help reduce lens thickness. My only other recommendation is to avoid polycarb lenses, unless optical quality is not at all a requirement. Thanks. I I'm in the UK and looking at a Specsavers own brand called Osiris. Model number is 548. What does aspheric do? Sorry, one more question. I hope the the thinner lens doesn't make the prescription look stronger? > Thanks. I > I'm in the UK and looking at a Specsavers own brand called Osiris. Model > number is 548. > What does aspheric do? In looking at the Specsavers pricelist: http://www.specsavers.co.uk/cgi-bin/strudwick.sh/s?langid=1&pfmt=1&siteid=39&pna me=options/thinandlight_prices.html I notice they have the following Hoya 1.60 lenses: Hilux Eyas 1.6 HV Aqua £140 - Spheric Nulux Eyas 1.6 HV Aqua £140 - Aspheric http://www.hoya.co.uk/index.php?SID=45a5880eb61b4220425593&page_id=3044 http://www.hoya.com.hk/products/plsingle/003.html#b The Asperhic will be a little bit flatter and a little thinner (quote from the Hoya UK site: "aspheric lenses (Nulux design) provide a thinner lens with a larger, distortion free field of vision"). The Aqua is a Hoya AR (anti-reflective) coating, which is probably why they are more expensive than the Pentax. I cannot tell from the price sheet what the Osiris model is, but I assume it is one of the Pentax lenses (index unknown). If you could get a Pentax 1.60 aspheric single vision lens, that would be the equivalent of the Hoya (but to compare prices accurately, make sure you know if the lenses include AR coating). > Thanks Mark A & Dom > [quoted text clipped - 5 lines] > I've noted your comment about not going above 1.60, that's helpful. So for > metal frames, you'd definately recommend. To know how many mm (or fractions of mm) your lens would be thinner is a calculation that requires your prescription including the astigmatism and axis, your PD, the frame size, and the index, centre thickness and base curves of the lens. It's not a simple thing so it's not done routinely at retail unless the practice has specialised computer software provided by the lens manufacturers (most don't, at least not here in Australia). Aspheric makes the lens a little flatter and it doesn't appear quite as 'strong' looking... but it probably doesn't make all that much difference for your prescription. It's great for higher prescriptions and especially for plus (hyperopic) prescriptions. A thinner lens (i.e. higher index) doesn't look 'stronger'. If you're wavering between conventional and high index then the Sola Spectralite lens may be a good compromise: it's 1.54 index, aspheric, high quality optics, but not that expensive. It may be called something else (rebranded) in the UK but it's an international brand. Dom > Is it possible to >find out what the difference between the regular and thinner lenses would >measure? Rx = -2.75 Cr39 (index 1.50) 2mm CT (center thickness) 50mm round blank spherical ET (edge thickness) = 3.8mm The following have 1.5mm CT, although in some frames this can be reduced to about 1.00mm to 1.2mm 1.53 (Trivex) 3.2mm 1.60 = 3.0mm 1.67 = 2.9mm 1.70 = 2.8mm 1.74 = 2.7mm As mentioned by others, the size, shape, and decentration will influence these numbers, as well as the CT required. I've used Trivex in low powers to reduce weight. For instance, a 50mm round blank, -1.00D Rx weighs 6.1 grams, Trivex (1.0 CT) is 2.9grams. Most folks can not feel the glasses when combined with a low mass frame like the Silhouettes (about 2.75 grams). Hope this helps, Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "An expert is a person who has made all the mistakes that can be made in a very narrow field." - Niels Bohr > Sure, the big numbers are -225 and -275. It also depends on how big the frame you've chosen is, and what it's made of. A small frame made of plastic (=thicker frame edges) conceals edge thickness much better than a large frame made of metal (=thinner frame edges). So if you're buying a big and/or metal frame then it's more worthwhile paying for high-index (thin) lenses. And, it *also* depends on your PD (= distance between your eyes)... a person with a smaller PD (ie. close-set eyes) will get more benefit from a high-index lens, all else being equal. Having said all of that, I generally start to recommend high-index lenses at around -2.50 to -3.00 ... but there are no hard & fast rules about it. Dom Sure, the big numbers are -225 and -275. Thanks Mark A & Dom. I'm planning to make my purchase this weekend and will ask about the 1.60 lenses. Do you think they'll offer them or will I have to make a point of asking? > Thanks Mark A & Dom. I'm planning to make my purchase this weekend and will > ask about the 1.60 lenses. Do you think they'll offer them or will I have to > make a point of asking? Who knows what they will offer? Ask them about what which 1.60 ashperic lenses they have, and ask them which ones have AR coating so you can make a fair comparison between brands/models. Do not let them talk you into polycarb (1.59 index). Be aware that some lenses may have cheap AR coatings that are not very durable and are hard to clean, so make sure you get the name of the coating (prefereably aplied by the manufacturer). If you are not careful with your lenses, and need to keep them for more than 2 years, and cost is a big factor, you may be better off without AR coating. From experience my guess is an intelligent conversation about brands might be difficult but I'll make sure it's not polycarb. I'd like to think the next lenses would last more than two years but that's not been my experience in recent years as I think my prescription is still changing. Even so I'll definately want to go for anti-reflective as I really don't like the effect without. You've been a great help, thanks. |
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