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Medical Forum / General / Vision / November 2005polycarbonate with a weak prescription
Hi, I have recently purchased some glasses from Shoppers Optical (their Maxthon brand), and have noticed that even though my vision is clear, they seem to make things appear slightly hazier, or dimmer. This is the first pair of glasses I have ever worn, and I have a fairly light prescription. <1 Sph in both eyes, but I do have a prism adjustment of +1.5BU and -1.5BD I have read that polycarbonate lenses have relatively poor image quality, and am considering going to cr39 or glass. The lady at shoppers optical said the quality of the lens shouldn't be an issue for a prescription as weak as mine. Will I notice a difference with cr39 or glass, or am I wasting my time? (keeping in mind my prescription strength) Also, are fogging issues a big problem with glass? I live in Canada, so below freezing temperatures aren't all that uncommon. Thanks, Todd Yeah, I doubt poly is the problem with that weak of Rx and I HATE poly. Wondering why you were put in poly for such a weak Rx though... Your problem (and I'm not a doctor or optician, just an experienced wearing) is that your lenses don't have an AR coating. Glasses without an AR coating reflect up to 10% of light, dimming your vision, making it hazy, and causing you to see reflections. The improvement an AR coating adds is amazing. Get some AR lenses (and they may as well be CR-39 if you don't have special safety needs, which, if you're considering glass, I'm guessing you don't) and I bet you'll be happy as long as it's a QUALITY, name-brand, vacuum chamber applied (or precoated lens) coating. Mark > Hi, > [quoted text clipped - 21 lines] > > Todd > I have read that polycarbonate lenses have relatively poor image > quality, and am considering going to cr39 or glass. [quoted text clipped - 4 lines] > Will I notice a difference with cr39 or glass, or am I wasting my time? > (keeping in mind my prescription strength) I can only speak from my experience, mildly myopic (-1.0 bilateral), no prism... glass lenses feel the sharpest to me (yes, I'm that old <g>), but in the upper midwestern US, they were a pain about fogging. Also managed to split a pair of plastic frames (this was mid 1970's) with differential contraction of glass and plastic by going from subzero air to room temperature and back repeatedly. And they're heavy. I wore glass lenses for years. My first pair of plastic lenses were a nice change in weight, and had lesser problems with fogging. Don't know what the composition was, probably CR39 -- mid 1990's. I know my most recent pairs (same prescription) are CR39 and polycarbonate safety glasses. I'd judge the CR39s to be *maybe* a smidge less sharp than my old glass lenses, but not noticeable in wearing. The polycarbonates make me feel like I'm underwater looking up... "swimmy" is the only way I can describe it, and they're noticeably heavier than the CR39s in a similar size lens and frame. The next pair of safety glasses will be Trivex, based on advice here (thanks, guys!) If I had to choose solely between glass and polycarbonate, I'd take the glass, hands down. Between glass and CR39, my nose thinks that CR39 is a very good choice, and my eyes seem to concur. FWIW, about $0.02. Kay Yeah, polycarbonate isn't very good... but I still think he's noticing the lack of AR more than anything (assuming he doesn't have AR) Your poly lenses in the same size frame will be substantially LIGHTER than CR-39 though. Poly has an index of 1.59 and CR-39 is 1.50, so you have less lens volume. It's also ground thinner in the center from what I understand = even less volume. Then, what volume you do have weighs less. Poly has a density of 1.22g/cm3 and CR-39 is 1.32g/cm3. In short, poly lenses are DRAMATICALLY lighter than CR-39. Therefore, the frame you had with the poly lenses must have been heavier.. Thanks for the advice. I do actually have an anti glare coating so I'm thinking the issue is either the lens material or my pickyness :) Based on Kay's comments it sounds like I might notice a difference with cr39 or glass with my prescription. I'm not sure why shoppers optical suggested poly...I've read in other posts that they make the most profit off of poly, so maybe that has something to do with it. I think I'm going to give the CR39 a shot, at least it's cheaper than the poly lenses. Thanks for the help, Todd Yeah, if you have AR then that's not the issue as long as it's decent. Poly is widely recommended for liability reasons and increased satisfaction with thickness and weight. The vast majority of people couldn't care less about optics. I think you'll be happier in CR-39, that's for sure. Make sure you still get an AR coat - that's the one biggest most important decision when buying glasses in my opinion. <cutiepieusa@gmail.com> said to somebody or other: > Yeah, if you have AR then that's not the issue as long as it's decent. > [quoted text clipped - 3 lines] > that's for sure. Make sure you still get an AR coat - that's the one > biggest most important decision when buying glasses in my opinion. Well, I care. What exactly is CR-39? I guess AR means anti-reflective. What exactly is that, these days? There's an ad on my mail table that says I can get it in Teflon. Used to be it was a layer of one kind of glassy substance over another. Perusing the Web, it seems that AR can cost $5 or $100 or whatever. Perhaps there is, somewhere, a white paper about how the various AR coatings take to the various lens substances, what their pros and cons might be, how much they cost, and how to save a buck without sacrificing too much quality. These things seem true: the cheaper the lens, the easier to clean it, and the greater the need to clean it carefully so as not to abrade it. What are the situations where AR makes the most sense (probably night driving/flying). Under what conditions could it be reasonably omitted? What are the whys and wherefores here? (Yes I know that ~ 4% is lost by reflections at each surface, and I know what lenticular flare is. Also I know that most things look pretty good through a window, regardless that said window is not anti- reflective.) Thanks -- Dicky > Yeah, polycarbonate isn't very good... but I still think he's noticing > the lack of AR more than anything (assuming he doesn't have AR) I don't know that I'd say that... my polycarbonates with AR are just as disquieting as polycarbonates without AR. Just may not be a good material for some of us who notice these things and don't seem to adapt well. > In short, poly lenses are DRAMATICALLY lighter than CR-39. Therefore, > the frame you had with the poly lenses must have been heavier.. Yah, I forgot about the side shields on the safety glasses. That's the difference, I'll bet. FWIW, I do prefer AR coatings, and at least some of them are pretty durable. My current pair of everyday CR39 are going on 9 years old, and still fairly unscratched, and I'm not the most gentle person in the world with them. Kay Trivex is a much better substitute for poly. I like the Hoya Phoenix material with their super-high vision AR coating. AR coatings need to be tailored for the specific lens substrate for maximum durability. Once you've had a high quality AR/hardcoat that is easy to maintain, you will never want an uncoated lens again. I don't presume or prejudge how much or how little people want to spend. I just know what it takes to give them the best possible vision. DrG > ... Once you've had a high quality AR/hardcoat that is easy to maintain, > you will never want an uncoated lens again. That would depend on what advantage the coating imparted, and upon its cost. Seems, at least in some cases, that the coating can cost more than one expects to pay for the whole array, including a fancy frame. The uncoated lenses are easy to maintain, and easier to clean apparently, in some cases, according to reports here at s.m.v. > I don't presume or prejudge how much or how little people want to > spend. I just know what it takes to give them the best possible > vision. How do you actually feel about the best possible mark-up? Well, no matter, my wife would like the best possible vision, even though she does not do any night driving. She gets new frames every time, and at least once a year, on general principles. Given enough time, she will no doubt find you, if you do not find her first. -- Dicky Dick, I am not a wholesaler. I am a retailer. I certainly hope that my markup isn't excessive, but that is indeed in the eye of the beholder. I like to think that we add considerable value to what we sell by virtue of expertise, product selection, etc. The better coatings are indeed more expensive. They are often done in high volume on stock single vision lenses, which makes it much less expensive. But, on a surfaced lens such as a progressive, the coating needs to be applied after the surfacing. You wouldn't believe some of the junk I see, which then becomes the yardstick they apply to me and everybody else. DrG More likely the 3pd of prism is causing the problem, not the material. Polycarbonate still gives pretty good image quality, as seen in many major brands of sunglasses. Dicky, AR coated lenses seem to be easier to get dirty, but in fact it is their high light transmission that highlights marks on the lenses. Use the proper lens cloth and they are very easy to clean. I don't think of the cost to patients, we sell all lenses with AR coatings as they are better, clearer and more scratch resistant. grant |
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