 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Medical Forum / General / Vision / August 20071.74 high index lenses vs. 1.67
I think that I am now in the -7.50 range in both eyes, and, without the Rx in front of me, I do not have the other values, but I do not think that they are terribly abnormal. The optometrist that I am going to is highly rated by a local consumer publication, but he (1) seems a little out of his comfort zone when contending with high index lenses, and (2) seems to be pulling prices somewhat out of the air. Whatever coating(s) is/are on my current lenses began to flake off catastrophically about a month ago. The lenses are about 3-1/2 years old. The same thing happened to a previous pair of 1.67 lenses, after about 2-1/2 years. He tells me that the coating on my current lenses is a "high index" coating, but not an AR or scratch-resistant coating. I am beginning to wonder, after spending an hour or so on this board, whether he really knows what he is talking about. He has done some checking with his suppliers (at my request, regarding the durability of whatever coating is flaking off), and has come back noting that I can get a pair of 1.74 lenses with an anti-reflective coating included, for about the same price ($15 difference) as a pair of 1.67 lens with some sort of Teflon coating applied (to the tune of about $100). The current lenses that I am wearing do not have this Teflon coating applied, but there is indeed some sort of coating that is flaking off both the front and rear surfaces of the lenses. I am, at this point, very confused about where I am and where I possibly should be headed. I have read on this board that the optical properties of the 1.74 lenses are not as good as the 1.67s. With the frames that I currently have, I am satisfied with the thinness of the lenses provided by the 1.67s. I am mildly bothered at times by the spectral dispersion at the periphery of the lenses, and I would guess that this would be worse with the higher (1.74) refractive index. Can anyone provide me with some advice about where to proceed from here? Or at least advise what questions I need to be asking about the lens choices in order to make an informed decision? >I think that I am now in the -7.50 range in both eyes, and, without > the Rx in front of me, I do not have the other values, but I do not [quoted text clipped - 35 lines] > here? Or at least advise what questions I need to be asking about the > lens choices in order to make an informed decision? If you are satisfied with the thickness and weight of a 1.67, then I would stick to that index. The higher the index, the worse the chromatic aberration (measured as abbe value- the higher the better). There are many lens manufacturers who offer models with a 1.67 index, and only a few offer 1.74. Not to mention that 1.74 is more expensive. A coating does not have an index (at least not a measurable one). If a coating had a measurable index, then a coated lens would have a different power than an uncoated lens, which is a bit ridiculous. What the OD may have meant is that higher index lenses generally are more likely to have a premium coating included, simply because the price of the high index lens is so high already. Anytime you buy a lens (just like when you buy a car) you need to know the lens manufacturer, and the model. You also need to know the material/index, and the coating (if applicable). For example, Zeiss is lens manufacturer who offers the Clarlet Single Vision aspheric lens design, in 1.5 or 1.67 index material, with a choice of several different AR coatings. Most of higher-end lens manufacturers offer a factory AR coating to a finished lenses which is usually more durable than a generic coating which can be applied to any lens at any lab. Find out the exact name of the coating and preferably get a coating that is applied by the lens manufacturer. Examples or some of the better AR coatings are Essilor/Varilux Crizal, Zeiss Carat Advantage, etc. Some companies offer several different coatings, so consult with the manufacturer website to see which is their premium coating. The advantage of the Teflon coatings is that they are easier to clean, and may be more durable in the long run for that reason. There are not many factory AR coatings that will flake off, but 3.5 years is stretching the limit of most AR coatings unless you are extremely careful. My optical background is not in eye care optics. Being retired for some time now, I am not fully conversant with current trends. Nevertheless, fundamental optical principles are still in force. Although there are advantages to plastic lenses, there also are disadvantages. Compared to glass, polymer (plastic) is less durable. Antireflection (AR) coatings on plastic cannot be as durable as hot deposited coatings on glass. Some optical coatings on military equipment are so tenacious that they cannot be removed without destroying optical surfaces. The most likely form of damage will be from abrasive particles, sand for example, during cleaning. From my own experience, it appears that dispensing opticians, who usually do not do there own coatings, are not capable of providing such hot coatings even if you do opt for glass. Most of them do not even know about hot coatings. On 8/2/07 7:28 PM, in article VcidnfTdFKJSDy_bnZ2dnUVZ_hWdnZ2d@comcast.com, > A coating does not have an index (at least not a measurable one). If a > coating had a measurable index, then a coated lens would have a different > power than an uncoated lens, which is a bit ridiculous. What the OD may have > meant is that higher index lenses generally are more likely to have a > premium coating included, simply because the price of the high index lens is > so high already. It is a well known optical fact that a light ray passing a stack of optical windows, each with parallel faces, does not deviate in direction. If the ray is oblique, the ray can get displaced sideways but its direction outside a window is unaffected. Simple optical coatings are so thin that even this sideways displacement is not observable for spectacle lenses. Even though the coating is deposited on a curved surface, the local geometry (over distances compared to wavelength of light) near a ray is such that the layers are effectively flat plates. > Anytime you buy a lens (just like when you buy a car) you need to know the > lens manufacturer, and the model. You also need to know the material/index, > and the coating (if applicable). For example, Zeiss is lens manufacturer who > offers the Clarlet Single Vision aspheric lens design, in 1.5 or 1.67 index > material, with a choice of several different AR coatings. I do not know, and I am willing to be enlightened, whether aspheric surfaces are used primarily to improve optical performance or to reduce protrusion of the lens in front of the frame. > Most of higher-end lens manufacturers offer a factory AR coating to a > finished lenses which is usually more durable than a generic coating which [quoted text clipped - 9 lines] > factory AR coatings that will flake off, but 3.5 years is stretching the > limit of most AR coatings unless you are extremely careful. I have had glass lenses with an AR coating consisting of single quarter-wave layer of hot deposited magnesium fluoride last the life (more than five years) of the prescription even with vigorous cleaning. Bill -- Fermez le Bush--less than 18 months to go. > I do not know, and I am willing to be enlightened, whether aspheric > surfaces > are used primarily to improve optical performance or to reduce protrusion > of > the lens in front of the frame. An aspheric lens is used to provide a flatter lens (and usually thinner and lighter). It requires a more precise fitting, since the pupil must be at the correct position relative to the lens optical center for optimal vision. For this reason, aspheric lenses are more useful in the higher indexes, and are usually not recommended for low power Rx's. >> I do not know, and I am willing to be enlightened, whether aspheric >> surfaces [quoted text clipped - 5 lines] >lighter). It requires a more precise fitting, since the pupil must be at the >correct position relative to the lens optical center for optimal vision. True. Off-axis performance is the same. There is less magnification/minification, and reduced spherical aberration in very high plus powers. >For this reason, aspheric lenses are more useful in the higher indexes, and >are usually not recommended for low power Rx's. It would be more accurate to say that asphericity is useful for higher powers. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman >My optical background is not in eye care optics. Being retired for some time >now, I am not fully conversant with current trends. Nevertheless, >fundamental optical principles are still in force. Dear Salmon, As I can see from you message you are professional in optics field. I have MS in Physics. One should not be eye specialist to understand how optical devises works And I afford myself to refer to the eye glasses and contact lenses as ‘optical devices’ I am not a big specialist in optics, but I am thinking that 1.Progressive maltifocals lenses should have enormous astigmatism and aberration And I doubt that someone use something like that in any technical optical devise that produces image. They use it for us, humans just because constriction eye-brain is amazing. 2. Multifocal contacts even worse (concentric zones with different optics power) Especially material from they made it from –polymers, silicon rubber and hydrogel composite– it should not have high optic properties, It should not give good concentric rings with correct power At least with wearing it should be tremendously deformed. Am I right ? And it gives two images on retina (I afford to mention here: two poor quality images _- Am I right ?) , it is job of brain to Reconstruct the picture. When I am working with any optical devise I know I can not tell it “wait , devise, you will Use to it.” How they tell it with progressives and multyfocal contacts” I know that optic should be high quality . I am thinking that it looks like high – scale experiment on human eye-brain system. Lena >I am not a big specialist in optics, but I am thinking that >1.Progressive maltifocals lenses should have enormous astigmatism and >aberration The unwanted surface astigmatism is approximately equal to the Add power. Segmented multifocals provide much larger fields of vision, usually with better acuity, especially in higher Add powers. >2. Multifocal contacts even worse (concentric zones with different optics >power) [quoted text clipped - 3 lines] >At least with wearing it should be tremendously deformed. >Am I right ? Right, although translating (with a line) multifocal CLs can provide excellent VA and visual comfort. >And it gives two images on retina (I afford to mention here: two poor quality >images _- Am I right ?) , it is job of brain to Reconstruct the picture. [quoted text clipped - 3 lines] >know that optic should be high quality . I am thinking that it looks like >high scale experiment on human eye-brain system. At least the PALs (progressive addition lenses) have zones of relatively clear vision, the simultaneous CLs present a pretty messy image on the retina, leaving the brain to sort it out. Others here have described it has junk optics. From an opticians viewpoint, the same could be said about PALs, although not to the same degree. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman >I think that I am now in the -7.50 range in both eyes, and, without >the Rx in front of me, I do not have the other values, but I do not [quoted text clipped - 9 lines] >old. The same thing happened to a previous pair of 1.67 lenses, after >about 2-1/2 years. It depends on the level of care they received, but in general, AR failures are rare with the newer coatings. The better coatings are not available on all types of lenses though. >He tells me that the coating on my current lenses is a "high index" >coating, but not an AR or scratch-resistant coating. I am beginning >to wonder, after spending an hour or so on this board, whether he >really knows what he is talking about. Not a clue. >He has done some checking with his suppliers (at my request, regarding >the durability of whatever coating is flaking off), and has come back [quoted text clipped - 4 lines] >Teflon coating applied, but there is indeed some sort of coating that >is flaking off both the front and rear surfaces of the lenses. Coating problems on ophthalmic lenses is a complex subject. Because you've had problems in the past, I would ask for Crizal Alize, Super HI-vision or HiVison with View Protect, the last two are from Hoya. >I am, at this point, very confused about where I am and where I >possibly should be headed. I have read on this board that the optical >properties of the 1.74 lenses are not as good as the 1.67s. Essentially the same. >With the >frames that I currently have, I am satisfied with the thinness of the >lenses provided by the 1.67s. I am mildly bothered at times by the >spectral dispersion at the periphery of the lenses, and I would guess >that this would be worse with the higher (1.74) refractive index. Nope. Abbe of 1.74 is 33, 1.67 is 32 (higher is better). 1.74 is about 10% thinner. There is a bi-aspheric/atoric version of the 1.74 that might be a tad thinner but may not be available in the US. Consider Hoya's 1.70 index- better optics than the above, for those sensitive to chromatic aberration. Better still is Trivex- it's thicker than hi-index but the weight will about the same. Robert Martellaro ~~~~~~~~~~~~~~~~~~ Optician/Owner Roberts Optical Wauwatosa Wi. ~~~~~~~~~~~~~~~~~~ "Science is a way of trying not to fool yourself." - Richard Feynman |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.