I just bought some rimless glasses, really nice ones but they have polycarbonate lenses which are very thin. I'd heard that polycarbonate was usually used for very strong prescriptions, mine's -1.75. I wonder if this is a common thing. Also does polycarbonate make the lenses look weaker? My old plastic lenses look stronger than these new ones. These are so thin (about 2.5 cm) they're almost fragile.
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For information regarding polycarbonate lenses, you may find the Polycarbonate Lens Council website to be helpful. There are a few articles there that will give you an idea why polycarbonate, a highly impact resistant material, is a good choice for rimless frames even though it is thinner than other materials.
Seiko's 1.67( (MR-10 Resin) lens is another good choice:I looked into the lens after reading the article in the Product Review Forum and we have been using it for our patients with higher powered prescriptions with great sucess.The same material used in polycarbonate lenses is found in helmet visors worn by astronauts during space travel and shields carried by riot police. The exceptional protection provided by polycarbonate lenses has made these tough lenses the only choice for children's eyewear. With the active lifestyle enjoyed by many adults today, wearing lenses made of polycarbonate adds a comforting margin of protection for your precious eyes. Polycarbonate lenses are the best investment you can make when ordering new eyewear.
http://www.polycarb.org/poly6.htm -
Finn:
Give us a better idea of what you mean when you say that your new lenses "look weaker". Are you specifically referring to the physical appearance (thickness) of the lenses, or has your vision changed in some way that they seem weaker when you look through the lenses?Comment
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I'm guessing you are referring to the fact that the lenses appear to be thinner than your old lenses (in other words, they appear to have less power than your old lenses because they are thinner).
Your new lenses are thinner because Polycarbonate has a relatively high index of refraction (compared to regular plastic). In other words, the material does a more efficient job of refracting light. Therefore, you need less of the material to do the same job. Also, polycarbonate is the second lightest material used for eyeglass lenses- so your lenses should feel rather light as well.
Regarding fragility, your polycarbonate lenses should prove extremely durable (i.e., resistant to breaking). Polycarbonate is one of the safest lens materials available. Of course, your lenses are still susceptible to scratching, so be sure to practice usual care when cleaning them.
Hope this helps... enjoy your new eyewear!Pete Hanlin, ABOM
Vice President Professional Services
Essilor of America
http://linkedin.com/in/pete-hanlin-72a3a74Comment
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To answer Susuan's question - I didn't mean that they lenses appeared weaker when I looked through them. Compared with my old pair that were regular plastic these look different. For example, if you were someone who was looking at me wearing them you'd notice that things appear to be a lot smaller through the old lenses than through the new ones. Hence my question that because they are thinner, do polycarbonate lenses give the overall effect of the prescription being less strong? I can understand the appeal of thinner lenses, but is one of the benefits for strong prescriptions to look less strong? If polycarbonate lenses are for the stronger prescriptions, what would they usually start at, and at -1.75 would you consider mine to be strong?Comment
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When I was dispensing glasses, polycarbonate was the standard lens. Plastic was almost un heard of in our office. When a patient purchased rimless frames, Polycarbonte was a must. Higher powers would go into a higher index like the 1.67 that was mentioned earlier. Now there is Trivex (awesome stuff). To answer your question I think your frame selection played more of a factor in deciding to go with polycarbonate lenses then a -1.75 rx. Meaning -1.75 isn't a terribly thick lens in regular plastic.Comment
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Jo - how to explain what I mean? No I don't mean that I think my eyes look smaller to someone looking at me. What I think looked smaller through the old plastic lenses was anything that was behind me, or seen through the lenses. I'm sure you know what I mean - when you get a view through someone's lenses from the front things behind look smaller. With the polycarbonate lenses the effect seems less somehow, the plastic definately make things seen through them from the front appear smaller. I guess that makes the prescription seem stronger to the observer. My -1.75s appear hardly to have any prescription at all compared to the old ones.Comment
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Perhaps your new lenses are aspheric. Aspheric lenses tend to have less minification/magnification. Actually, the benefit is usually less noticeable on minus lenses (like your own), but you may be noticing the improvement.Pete Hanlin, ABOM
Vice President Professional Services
Essilor of America
http://linkedin.com/in/pete-hanlin-72a3a74Comment
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Finn:
There are in fact a variety of factors that go in to producing the "power" of a lens. Those factors include the "index of refraction" of the lens material, which refers to how well the lens material (all by itself) redirects light; the "curve" of the lens on the front and back surface (which is ultimately dictated by the "index"); the "thickness" of the lens (more of a factor in plus power lenses); and "vertex distance", which is how far the lenses sit from your eyes when wearing your glasses (more of a factor in high power lenses).
I think that each of the answers in this thread plays a part in why your lenses look so different. First and foremost, polycarb has a higher index of refraction than your former plastic lenses, therefore less curve is required to produce the same lens power. That is what makes high index lenses (like poly) thinner. Also, polycarbonate is a stronger material and can safely be ground thinner at the centers than plastic (though that may not be a factor here because outside the U.S., I don't think other countries have federal laws that govern adherence to "impact resistance" guidelines).
The eye size of the frame plays a big part in thickness and as mentioned before, if your new frame is smaller, your lenses will be thinner (even without high index), and might also change how strong they 'look'. By size, I am referring to how big 'around' the lenses are in the frame, or how wide the lenses are across at the widest point in the frame eyewire. With your type of prescription, the lenses are thinnest at the very center, increasing in thickness from there out to the edge; so the smaller the eyewire, the thinner the lenses will be.
Lastly, based on your description I suspect (as did Pete) that your lenses are likely an "aspheric" design. This feature refers to lens curvature, not the material. Aspheric designs use flatter curves on both the front and the back surface of lenses. They further reduce lens thickness, and in some cases noticeably reduce how strong the lenses look on your face. In general, aspheric lenses (particularly in stronger powers) make everything look more 'natural'; not just when people look at you, but the images that you see when wearing aspheric lenses will look more natural also (images are less minified or magnified in plus powers).
That enough brain exercise for the day? After learning everything from this thread, you could probably go apply as an apprentice optician!Comment
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Another factor to consider is that your prescription possibly changed at the last visit. Your new lenses are at the cutting edge of technology and as long as your vision is good, the thinness is a "plus" so you look better :0). It is not uncommon to use polycarbonate for your Rx. In fact it is highly recommended for children because of it higher impact resistance and the corresponding increase in safety. You can be a walking advertisement for the "newer and better" materials that are being introduced.Comment
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Thanks to you all for your comments. I think you're right that my lenses are also aspheric as they seem very flat as well as very thin. And I mean flat, like completely flat, till now I've always had lenses with a bit of a curve at least. Anyway I like to think I'm at the cutting edge of technology, that's good, and I really like the glasses themselves. I just wish people who see them could appreciate just how hot they are!Comment
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