From: JBag on
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?

From: Mark A on
"JBag" <thewiseinvestor(a)yahoo.com> wrote in message
news:1186096405.621870.305940(a)j4g2000prf.googlegroups.com...
>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?

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.


From: Salmon Egg on
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(a)comcast.com,
"Mark A" <nobody(a)nowhere.com> wrote:

> 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
> 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.

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.


From: Mark A on
"Salmon Egg" <salmonegg(a)sbcglobal.net> wrote in message
news:C2D84FB2.8EE4B%salmonegg(a)sbcglobal.net...
> 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.


From: Robert Martellaro on
On Thu, 02 Aug 2007 16:13:25 -0700, JBag <thewiseinvestor(a)yahoo.com> wrote:

>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.

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
>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.

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