Thinnest possible lenses?
Richard Davis
glasses. Are they polycarbonate or high index plastic? Are they
"aspheric"? If someone could even mention a specific brand name, I'd
appreciate it.
My prescription is -16.50 +2.00 x90, -16.00 +2.50 x90.
Thanks in advance.
(And thanks to all who answered my laser surgery question. Extensive
research has put the fear of GASH into me.)
Max Langfield
<davi...@earthlink.net> writes
get used to the peripheral vision, maybe 7-10 days. Chromatic aberration
with these high indicies will give some coloured fringes around bright
illuminated objects. These effects are adapted to after a short time.
Full aperture high index glass 1.8 or even 1.9 will be thinner than
other materials. However, with your presription you could consider a
blended lenticular such as a Rodenstock Lentilux. This is a high index
glass lens available up to -24.00 with an edge thickness of approx 4mm
at a diameter of 70mm.
The lens has a centre optical portion, the area of which reduces
slightly with increasing lens power. The outer part of the lens is
blended to reduce the thickness, but obviously this means that you will
not see clearly through that part. This is not nearly such a
disadvantage as you might think because all people that look through
high minus (-16.00) lenses will have a large field of view anyway.
With all lenses that you might consider it is essential that you have
them supplied with a good anti-reflection coating. The greater the
refractive index then the greater the surface reflection, unless they
are AR coated.
A good thing to do would be to visit some opticians stores and ask to be
shown examples of various lenses that they might recommend. They won't
necessarily have your exact prescription as an example, but they ought
to be able to show your comparisons of different lens forms in various
powers.
Good luck
--
Max Langfield
William Stacy
optimized for your pupillary distance and head size. Fortunately, the
new small eyesizes are "in" right now. I just got a pair of Lanvin
frames (Logo 1223) that have an eyesize of 41 and bridge 25. I've NEVER
had lenses this thin. With my p.d. of 64, each lens only needed 1 mm
decentration, and the endpieces are large enough to accommodate my
(somewhat big) head. I thought I'd see a lot of frame, but after an hour
or so, I don't.
BTW, they get mixed reviews, from "far out man!" to "Jumanji dude", to
"hey gramps".
But since I AM a grandad, it's ok by me.
Bill
dmi...@macconnect.com
> I'm trying to find the thinnest possible lenses available for my
> glasses. Are they polycarbonate or high index plastic? Are they
> "aspheric"? If someone could even mention a specific brand name, I'd
> appreciate it.
>
> My prescription is -16.50 +2.00 x90, -16.00 +2.50 x90.
>
> Thanks in advance.
>
> (And thanks to all who answered my laser surgery question. Extensive
> research has put the fear of GASH into me.)
Polyurethane lenses give a combination of decent optics and the thinnest
and lightest possible lenses. Some brands off the top of my head: optima
hyperindex (1.66 index) and sola vizio (1.66 index). Talk to some optical
shops. If you mention these brands and they act like they've never heard
of them, walk out. At a good optical shop mentioning hyperindex
polyurethane lenses should get them talking about a variety of available
lenses and what might work best for you.
best of luck,
david
Brad Rogers
>
>Polyurethane lenses give a combination of decent optics and the thinnest
>and lightest possible lenses. Some brands off the top of my head: optima
>hyperindex (1.66 index) and sola vizio (1.66 index). Talk to some optical
>shops. If you mention these brands and they act like they've never heard
>of them, walk out. At a good optical shop mentioning hyperindex
>polyurethane lenses should get them talking about a variety of available
>lenses and what might work best for you.
>
Have you looked at the the prescription?
1.66 mid index plastics are _not_ going to be the thinnest for a
-16.50D (unless they are a blended lenticular - and even then they
will look quite gross).
Stick to Glass lenses 1.8 or 1.9 in full aperture (if they are
available - I haven't checked). Or a blended lenticular in high index
glass (like Rodenstock Lentilux).
Remember that if the eye of the frame is small enough, even glass
lenses might work out quite lightweight.
Brad
William Stacy
> Oh one last thing , an AR coat should be considered for any high minus RX
> or high index material
Especially if you're grinding bi-concave like you described. Those real
images sitting out in front of bi-concaves (from focussed reflections
off the front surface) can be pretty disconcerting to the person looking
at the wearer!
Bill
Specs31
>available - I haven't checked). Or a blended lenticular in high index
>glass (like Rodenstock Lentilux).
>
>Remember that if the eye of the frame is small enough, even glass
>lenses might work out quite lightweight.
Couple of problems there...if this person is in the U.S then you have to
adhere to the ANSI standards...the 1.9 glass WILL NOT pass a drop ball
test!!...I won't make a 1.8 or 1.9 glass lens because you're asking for a law
suite. Another problem is you can not heat treat a 1.9 glass lens so the person
is walking around with out a tempered lens...
A more vialble answer if wanting to get creative is to flip a high index
lens around and surface the front and make a con-cave lens.
I do alot of low vision work for Ophthamologist and do this freaquently for
low vision problems.
A -14 on a flipped plano base lens will mean you would be cutting a -9 or
so on the front and the moled curve will carry partial power.
This creates a thin lens even in the higher minus powers....and if the
frame is kept as small and round as possible to help with decentration then you
can get a fairly wide field of abberition free zone.
Mixing a higher index (i,e, 1.60,1.66,1.59) in conjuntion with cutting a
front surface will help two ways....the molded surface will carry partial
power, the index of refraction will call for a flatter tool to get the same
power, A experienced optician should be able to help with the frame selection
to get the best results, and by not getting to steep of a molded curve (staying
in the -4 range) the tunnel vision should be kept to a minimum.
I did a pair of -17 (with some cyl.) for a 14 year old girl a couple of
days ago and ended up with a edge thickness of 6.5mm!!...thats thin for that
RX!!
This technique maybe not used industry wide but any good experienced lab
tech in low vision should be able to whip this job out as quickly as a regular
lens...
Oh one last thing , an AR coat should be considered for any high minus RX
or high index material...due to loss of light transmission due to index of
refraction can be compensated through the application of an anti reflective
coating. (fresnal's equation)
Jeff
eric pearson
Are you saying that consumers not needing a strong prescription are
now resticted by this ANSI *CRAP* to plasic lenses which scratch when
you breathe on them? If my LASIK brings me to 20/40 and I want 20./20
I am sentenced to the scratch-if-you-look-at them spectacle lenses
sold by Lenscrafters, etc? If I want glass lenses in my spectacles I
need to leave the US??? Who died and made that Commie idiot Nader in
charge>?????
regards,
eric pearson
er...@nospammindspring.com
Jkumar167
>now resticted by this ANSI *CRAP* to plasic lenses which scratch when
>you breathe on them? If my LASIK brings me to 20/40 and I want 20./20
>I am sentenced to the scratch-if-you-look-at them spectacle lenses
>sold by Lenscrafters, etc? If I want glass lenses in my spectacles I
>need to leave the US??? Who died and made that Commie idiot Nader in
>charge>?????
>
>regards,
You misread the post, or didn't understand the optics/language involved. The
poster was trying to say that to make the center thickness thin enough to make
the weight tolerable in a -16.50 glass lens, it would be too thin for US ANSI
standards. In a small to moderate Rx lens (such as you would use after LASIK),
the weight won't be as big an issue, so if you insist on glass you will be able
to get it no problem..
Max Langfield
<spe...@aol.com> writes
>>Stick to Glass lenses 1.8 or 1.9 in full aperture (if they are
>>available - I haven't checked). Or a blended lenticular in high index
>>glass (like Rodenstock Lentilux).
>>
>>Remember that if the eye of the frame is small enough, even glass
>>lenses might work out quite lightweight.
>
> Couple of problems there...if this person is in the U.S then you have to
>adhere to the ANSI standards...the 1.9 glass WILL NOT pass a drop ball
>test!!...I won't make a 1.8 or 1.9 glass lens because you're asking for a law
>suite. Another problem is you can not heat treat a 1.9 glass lens so the person
>is walking around with out a tempered lens...
>
the land of milk and honey, there is a small but meaningful advantage in
being able to view the world through optically wonderful lenses.
Seriously though, I did not realise that such safety constraints apply.
This is a shame because 1.7 Lentilux and 1.8 full aperture are superb.
I can understand that you are not a fan of 1.9, neither am I but it does
have its uses.
> A more vialble answer if wanting to get creative is to flip a high index
>lens around and surface the front and make a con-cave lens.
> I do alot of low vision work for Ophthamologist and do this freaquently for
>low vision problems.
> A -14 on a flipped plano base lens will mean you would be cutting a -9 or
>so on the front and the moled curve will carry partial power.
> This creates a thin lens even in the higher minus powers....and if the
>frame is kept as small and round as possible to help with decentration then you
>can get a fairly wide field of abberition free zone.
> Mixing a higher index (i,e, 1.60,1.66,1.59) in conjuntion with cutting a
>front surface will help two ways....the molded surface will carry partial
>power, the index of refraction will call for a flatter tool to get the same
>power, A experienced optician should be able to help with the frame selection
>to get the best results, and by not getting to steep of a molded curve (staying
>in the -4 range) the tunnel vision should be kept to a minimum.
> I did a pair of -17 (with some cyl.) for a 14 year old girl a couple of
>days ago and ended up with a edge thickness of 6.5mm!!...thats thin for that
>RX!!
With your restrictions as detailed above I can understand why you do
this, but are you sure that the field of view is stable enough?
Would you still use this technique for -23.50 DS. I supplied this lens
last week and it looks terrific as a 1.7 lentilux
> This technique maybe not used industry wide but any good experienced lab
>tech in low vision should be able to whip this job out as quickly as a regular
>lens...
> Oh one last thing , an AR coat should be considered for any high minus RX
>or high index material...due to loss of light transmission due to index of
>refraction can be compensated through the application of an anti reflective
>coating. (fresnal's equation)
>
>Jeff
--
Max Langfield
Workhorse
--
Michelle Denise Gentile
I do not speak for GlaxoWellcome.
You mean like the Big-Head-Little-Head syndrome? 8^)
Joe Fischer
: Hello!!!
: Are you saying that consumers not needing a strong prescription are
: now resticted by this ANSI *CRAP* to plasic lenses which scratch when
: you breathe on them?
Before my eyes started changing every few months,
I always bought glass lenses, mostly because I didn't
want to bother wetting the lenses before cleaning.
But after using plastic lenses for about three
years, I find they don't scratch all that easy.
ANSI stands for American National Standards
Institute, and they do actual tests, and set standards
for both safety and convenience of various utility.
So, if I needed high minus lenses, which are
very thick at the edges and thin at the center, I think
I would want to follow the standard, even though a ball
drop it a rather harsh test.
: If my LASIK brings me to 20/40 and I want 20./20
: I am sentenced to the scratch-if-you-look-at them spectacle lenses
: sold by Lenscrafters, etc?
I am neither a doctor nor optician, but I think the
same plastic lenses are sold by all dispensing vendors,
I have been seeing CR-39 on receipts, and seen the good
posting experts use the same term.
They don't scratch that easy, and if you need a
plus prescription, or a minus prescription less than
2 or 3 diopters, there should be no problem getting
glass lenses that meet the standard, as long as glass
lenses are not discontinued by the whole industry,
which I expect to happen, simply because plastic has
become better than glass for most uses.
: If I want glass lenses in my spectacles I
: need to leave the US???
No, but it is getting more difficult for me to have
them made, my favorite optician went out of business, and
my closest eye care center employs an excellent optician
that refuses to work with glass because he cut himself
too many times, meaning I have to wait longer for them
to have another location make the glass lenses.
I now think plastic lenses are preferable for
most people, the safety issue is paramount, and even
a tempered glass lens may not give the protection of
some plastics.
And simply wetting the lens or cleaning cloth
or paper towel should give the lens a reasonable
lifetime, I would be tickled to have to buy new
lenses every five years for the reason that they
are scratched, if only I can go that long without
needing a different prescription.
Regards,
Joe Fischer
Specs31
>this, but are you sure that the field of view is stable enough?
The key is to not get carried away with the factory molded side when
splitting the curves...if you maintain a outside curve of around 4Diopters
(give or take) with a vertex of 10 to 12mm then I have had great success with
outer edge periphral distortion not becoming a problem and the field of actual
lens surface needed to have the same amount of visual field is reduced...that
and making sure the fitting in monocular PD's and fitting optical centers helps
a great deal...
The main problem if you have a greater concave curve of 6 is tunnel
vision...but then again most people wearing the higher minus RX's are used to
some of this already in the majority of cases , if fitted correctly, I usually
am able to increase the visual field.
Jeff
Specs31
>now resticted by this ANSI *CRAP* to plasic lenses which scratch when
>you breathe on them?
The ANSI standards (which you like to call "crap :-) does not put limitations
on just making plastic lens..(cr39) and the plastic lens are not "scratching
when breathed on"...maybe a poly is though :-)...really if a lens is coated
after processing then it will last just as long as any glass lens.
>If my LASIK brings me to 20/40 and I want 20./20
>I am sentenced to the scratch-if-you-look-at them spectacle lenses
>sold by Lenscrafters, etc? If I want glass lenses in my spectacles I
>need to leave the US???
No...you can still get glass in a 1.60 high index as well as crown and PGX PBX
solid tint and regular 1.523....they just draw the line in the higher ondexed
lens like 1.8....
Me personally I consider my eyesight worth a little more then just running
willy nilly around in something I find unsafe...they take glass lens to a 1.0mm
center in over sea's but I find that pushing the envelope to far...next time
you visit your Ophthamologist ask him/her if they have any video's on
procedures...I know I have seen a few cases over the years where I sat in when
one of the OMD's I know was removing slivers of glass from someones eye...it's
not a pretty sight (pun intended)
I consider my vision very important to me...if I find that I can increase
the saftey factor by using something as close to the thinnest but safer...I'm
landing on the saftey side of that choice everytime
Oh and the ANSI crap does not just allow the issue we are talking about it
covers a wide range of problems that have to be fixed followed and
maintained...(ie. power ,prism,types of thickness,types of materials,frames.ect
ect ect.) some of them are kind of knit picking pains in the rump but as a
whole it does do the job of trying to maintain a indusrty wide set of rules so
that you should atleast get the same basic rules no matter where you get the
lens done.
Ta,
Jeff
Brad Rogers
I usually agree is everything you post (as the bredth of your
knowledge is obvious) but to suggest that a biconvex lens will produce
a "fairly wide abberation-free zone" I find this an incredible thing
to say!
On these high (minus) powers the best form of lens is a meniscus or
flat form. If we take that -16.50 Rx as before, the person wearing
that (with, say a -7.50 front curve and a -9.00 back), will be looking
through oblique astigmatism of about _6.50 D_ when looking only 15mm
from the centre!. Also, distortion _increases_ in this
biconcave.(Rotational distortion is about twice that of a flat). The
result of this is that the patient will have a clear central zone of
about _9mm_ diameter, beyond which the lens is optically useless.
A plano-concave on the other hand will give only about 0.25D of
oblique astigmatism at 15mm from the centre. This means better all
round vision. Also both ordinary and rotational distortion are less.
But - much thicker.
The real advantage of the Bi-con is, of course its thinner (as you
have indicated) but it is definitely at the expense of its optical
performance and its appearance.
Could it be possible that the low vision patients were not able to
tell the difference between the central and peripheral performance of
the lens?
Regards
Brad
Specs31
If you read the post you might have missed the part where I stated that
using a flatter nominal curve (2 to 6) depending on the material and maker
this curve changes...
But here's the thoery I go by...say the person getting the lens are a -15
sph. OU
(keeping this simple for debates sake) and has been wearing a PL.
lens...depending on the material index you could have anything from a -14.87
down taking into account eleiptical error....now what type of visual field do
you figure the person has been wearing or used to??...with a cut that steep the
distortion begins fairly quickly as you leave the OC...NOW if I cut a con-cave
with a -4 and split the difference you'll have less curvature on the occular
side so even though it would appear on the surface you are increasing
distortion by splitting plains you have actually increased the zone by having
less occular curve and nuetrilizing that some what with the front curve....on
the face value you would think that the lens would be just the opposite but the
key is to having less curvature and increasing the index some to allow for a
flatter occular curve is still maintain the correction of power....
I have tinkered over the years with this time and again..increasing power
split...
I've tryed it (just for experimental curosity)
a few ways ....
PL. with the needed correction(occular)
25% 75% split
35% 65%
50% 50%
When we set down and did it time after time the two worst in distortion
were the PL. and the 50%/50%...the latter I expected but the first was
not....splitting it around the 25% to 30% to 75% /70% split with a higher index
and anti reflective coating in the powers over -12 or so made a markable
difference to the person wearing the lens...
The trick we always found was keeping a chart that gave us the different
indexes and had the factory molded curve available to help us decide the best
split....namely you have to think about it where habit says grab a PL...but
take a Spectralite and measure the occular curves on a PL /1.00/3.00 all
nominal curves but the back curve is all the same a 3.00 :-)..of course we lose
some of the value by taking away the asherical cut but it does still keep it
higher then say If I used another choice in lens design and material
But if choices are made then splitting it on say a molded aspheric dbl
concave lens from Optima (1.66 abbe 32) then it also had advantages in certian
situations.The Optima is available in a -2/-4/-6 nominal curve but I usually
end up with using the -4 for the real crazy stuff because of the center
thickness of 18.5mm gives me some lens to work with while the -2 nominal curve
has only 12mm to get to grind the power into..this is fairly goodfor the -12 to
-17 or so range . I used the -6 a few times but that was under extreme
conditions.....any nominal curve past that (-6 is pushing it) defeats the
purpose, atleast for me.
I'm trying to wgh. the cosmetics against the function..,,,I try to lean more
towards the functionality then cosmetic...what the hell good does it to look
good but not be able to see!! LOL ..
If the con-cave curve gets above the 4 range then I started to see it
effected the distortion noticably...
Doing it so many times I have learned to juggle the different things like
index of material, abbe value and curve to get the best combination.....enough
rambling lets go over some of your statments and questions
>On these high (minus) powers the best form of lens is a meniscus or
>flat form. If we take that -16.50 Rx as before, the person wearing
>that (with, say a -7.50 front curve and a -9.00 back), will be looking
>through oblique astigmatism of about _6.50 D_ when looking only 15mm
First it's never that simple to split the powers (maybe you was doing it to
make it understandable to the laymen in the group) you have to consider
eliptical error here as well as the index of refraction effectiong the amount
of occular curve...I would never be placing it on a CR39 more likely a mid
index or in this case probably the molded -4 nominal aspheric lens from
Optima...so you just reduced both of those curves (nominal/occualar) by -3.50
on the nominal and increased the occular only by maybe a diopter or so
occular(allowing for index of 1.66) so we just reduced the oblique astigmatism
and increased the central zone :-)
>Also, distortion _increases_ in this
>biconcave.(Rotational distortion is about t>wice that of a flat). The
>result of this is that the patient will have a >clear central zone of
>about _9mm_ diameter, beyond which the >lens is optically useless.
Couple points here...the corrected acuity we are usually working with here
is more often then not 20/20 so there is somewhat of a area to play
in..secondly consider the periphral they had before wearing a occular curve of
(usung the example) -16.37 or so considering index and eliptical error..you
think that was a fish bowl? :-) now ontop of that lets cut up the eye a little
here (Bill will help me out if I make mistakes here:-) the fovea
centralis...lets go by memory but a 5mm deviation here and your best correction
is around 20/40 and it falls steeply from that point onward...now on top of
that with a myopic RX in this range peripheral zone is almost a distant memory
(or atleast a non distorted area is way out in left field :-) so here we
are....see where I'm at?
>Also, distortion _increases_ in this
>biconcave.(Rotational distortion is about t>wice that of a flat). The
>result of this is that the patient will have a >clear central zone of
>about _9mm_ diameter, beyond which the >lens is optically useless.
When you mix in all the influences, power,
index,nominal,occualr,corrected acuity THEN thats when I usually make some
judgment on what will work best where and how...thats one thing about
optics...each time I do these things usually I have to do something slightly
different to get (atleast in my opinion) the best possible result for the I'm
working with ....
It's not always the answer but I just like to have a few tricks under my
hat when the need arises :-)
We are always batting all this back and forth around the lab and it does
make it more interesting that all my accounts are strictly OD's and OMD's only
so I get to be more creative and see a very wide range of problems and not just
the simple stuff.....I have been pushing some of the OMD's I work with who have
this thoery that the best correction is magnificatio to get correction!!LOL and
have been tinkering with compund prism to help in macular problems....you
should set in some of the meetings we have (polite way of saying yelling
matches) :-) get a couple of Ophthamologists a n OD or two and a retinial
specalists and me in the middle and watch the
action begin!!!LOL
I have been getting some great results using an indirect and prism to get
some of these older people who was stuck in the 20/200 range (or worse) and had
been tryed to be corrected with plussing the heck out of them and I'll talk the
OMD's to letting me cut power by miles and use compound prism and now the can
see alot better....tough part is I just don't have as much time to spend
working with a trial frame and lens and scope to keep me happy!!LOL
Low vision is not for everyone, and regrettably opticians and techs coming
up through the ranks now adays seemed lost most of the time (no general insults
meant :-) I'm just speaking for the area I'm in only.
Oh well...learn something new every day and life will never get
tedious....
Who knows maybe someone will take something we all contribute here and
figure out even a better way to accomplish it...just hope they post it here so
I can use it as well!!!!
I guess this is a long winded post but feel free to correct or add to it or
atleast the parts that need added to or corrected....always willing to pass
information back and forth... Hopefully you won't take this as a "yea...but I
know this works and you don't know yada yada yada"...just explaining it a
little more indepth. from my experience..maybe the laymen in the group won't
appreciate the technicle stuff...but atleast I find it interesting :-)
THE END ....:-) (maybe ha ha)
Jeff
SpecOPTICS
Forgot to add something, another factor to figure into the equation is
vertex distance...
Going back to the EX: we were discussing a -16.50 OU...if the refraction was
done at 14mm and the wearing vertex distance could be reduced to 12mm then you
would shave off another .53^ of power to still get the corrected acuity...the
reson why I figured it this way is to get this solution....
A -4 nominal curve with a index of 1.66 will now make it a -9.00 occular
curve!! ...so we still get the same occular curve you mentioned in your post
but reduce the total amount of nominal curve from a -7.50 down to a -3.88 :-)
Jeff (spe...@aol.com)
Brad Rogers
Hi Jeff
>Hopefully you won't take this as a "yea...but I
>know this works and you don't know yada yada yada"
No not at all ! I think your contribution to this N.G is always very
high, your knowledge of lenses great as is your inginuity of lens
manufacture. Like you I am always open to learn something new.
There are however, some areas where your hypotheses are misguided and
based on subjective experience and assumption. With respect, I
believe this may be one of them.
Firstly, lets agree on a few things;
>the fovea centralis...lets go by memory but a 5mm deviation here and your best correction
>is around 20/40 and it falls steeply from that point onward...now on top of
>that with a myopic RX in this range peripheral zone is almost a distant memory
>(or atleast a non distorted area is way out in left field :-) so here we
>are....see where I'm at?
No I don't. You seem to be implying that the "field of view" of a
spectacle lens only needs to be as wide as the patients' central
vision! This is the same as "blacking out" a spectacle lens, leaving a
clear 3mm central spot around the optical centre. If this was all that
was needed from spectacle lenses, you would have to turn your head or
your entire body to see around you!!
The maximum field of view a person gets from a spectacle lens is the
furthest the eye can rotate and see clearly! How much the eye can see
clearly when it is "looking around" is affected by three main
abberations;
Chromatic abberation
Distortion and
Oblique astigmatism
Which are influenced by;
Refractive index (and V value)
Centre thickness
Back vertex distance
The angle of incidence (i.e the angle at which the eye is "looking")
and the FORM of the lens
I know you know what chromatic abberation is, but I would be
interested to hear what you think "distortion" is.
I understand distortion to be split into two types;
Ordinary distortion (gives the wearer a "fishbowl" or "pincushion"
image) and
Rotatory distortion which is caused by the _apparant_ movement of
images in the peripheral field - and this is what sometimes gives
people nausea when they try a new pair of glasses for the first time.
In both cases, the brain usually CORRECTS the distortion (so the
"bending" and the nausea go away after a few days).
Oblique astigmatism is the big one (please excuse me Jeff as I reckon
you know all this anyway!). This is the effect caused as a result of
the eye looking obliquely through the (edge of the) lens, and is
affected by the form (or design) of the lens. A -6.00 for example may
be that power when looking through the centre, but on looking 30
degrees away (approx 15mm from the optical centre), it may become
-6.00/-3.00DC ! The net effect would be "centre clear - edge blurred".
The field is therefore not good - much less than 30 degrees.
> But here's the thoery I go by...say the person getting the lens are a -15sph. OU
>(keeping this simple for debates sake) and has been wearing a PL.
>lens...depending on the material index you could have anything from a -14.87
>down taking into account eleiptical error....now what type of visual field do
>you figure the person has been wearing or used to??.
. . . . From Trigonometrical Ray Tracing;
Assuming n=1.66/ BVD12mm/C.T=2.0mm
Deg Rotation Oblique Ast(DC) Ord' distortion(mm)
10 0.07 0.153
20 0.26 1.433
30 0.56 6.573
40 0.93 28.056
Well, at what point do YOU think the vision is going to be
unacceptable? At 40 degrees (i.e about 26.5mm from O.C) the Rx becomes
-15.00/+0.93DC. I think thats pretty good for a -15.00!
Now lets look at the performance of a biconcave with say -4.00D
(nominal) front curve with 2.0mm centre subs and -11.019D curve on the
back surface (n=1.66/BVD12mm)
Deg Rotation Oblique Ast(DC) Ord' distortion(mm)
10 0.24 0.177
20 1.02 1.692
30 2.67 8.199
35 4.19 17.645
So you see that the amount of astigmatism caused by the rotation of
the eye behind the lens is much higher - in this case about 1.00D of
OA at 20 degrees (10mm from the centre) with the bicon', but the eye
can get BETTER vision with the flat, looking _twice_ as far (0.93 @
40deg). I will concede with this example that the bicon' gives less
distortion than the flat but would point out that distortion is
irrelevant as the brain corrects it anyway and with the biconcave, the
wearer would have such blurred vision around the edge they would't
notice it anyway.
In any case the biconcave design gives a lower field than the flat -
not greater.
I hope I haven't misunderstood your post and am doing you an
injustice, but I have spent a LONG time studying your reply and as far
as I can see it amounts to the same thing - you are getting a
semi-finished lens, reversing it on the generator and making a
biconcave lens. If this is so then I stand by all I said in my
previous post.
I'll now step off my soap box as I see that thousands of people around
the world have fallen asleep in front of their computer screens!
Cheers!
Brad
Specs31
miscalculated the math.....
>Now lets look at the performance of a biconcave with say -4.00D
>(nominal) front curve with 2.0mm centre subs and -11.019D curve on the
>back surface (n=1.66/BVD12mm)
>
>
Hopefully here you aren't thinking this curve combination with index of 1.66
and a Vrt of 12 mm is giving you a power of -15 :-)
A 1.66 index with a nominal of -4 (well correctly stated as a +4 :-)
and a -9.00 occular
curve will give me a total combined power to get the -15 correction ....the
power you are listing would be in the -18 range.....but thats comparring apples
and oranges really..:-)
Basically what I have seen over the years using every text I've read and
formula from here to there do not always translate to the real "world" as well
as to each complication..
I took into account the oblique astigmatic effects (in my opinion) the
most telling factor when dealing with a high myopic RX...an have noticed over
the years that even though (as you stated correctly) in your equations that the
person wearing the lens have adapted to the problems the incurr from wearing
the RX...they are used to the warping of lines in the periphral,the distortion
from the refracted light from the sharp angles.....so even though
theorectically you stated the facts I have had great success in converting alot
of those high myops into other ways of cutting (like the bi- concave) and in
the highest % probably in the 50% range they have said they could tell NO
difference optically from what they had to what they have :-) other then
cosmetically they were alot happier.....the other 50% (give or take) split
evenly between "I can see better" to "this is worse"(probaly higher the
25%)....those are hard numbers I can live with...the ones that this did not
work for (remembering I do this only in case of severe myopia) I place back
into a Plano curve......
Even though you may have copernicus on your side :-) I still have A high
number of satisfied people who did it through a mean comparison...they knew how
the seen before the change...and knew how the saw afterwards.... all I can say
is that theory and equation do not always meet "real" solutions...
My best defense is that even though you may have two people with the
exact same RX...the real actual physic's of the way the acuity is between the
two may differ......
I have had people who are ave. in refraction in the +2 range where one
can wear just about anything (material & design) and a second can wear only
glass (crown) without have complaints on distortion...why(?) I don't know
exactly....but I have learned there are no "set" answers to fit everything
across the board....
Before going ..one other point I might throw out there and see if anyone
else has seen a pattern...in a great number of high myopia people that I have
worked with over the years have come in using less of that field of rotation
then you discussed...and the higher the myopia the more tunneled it
appears(either naturally or learned)....one of these days I might start keeping
a mannual and having the OMD I work with alot try to actually help me plot it
out exactly on a visual field...I think possibly thats another reason that even
though mathmatically you point out some rather large discripencies in the
oblique astigmatism I still have a rather high aceptable rate of getting them
to work using the bi-concave cuts....
Mathmatically I could write it out from point A to point B ..BUT the
actual physical charactiristics don't match the theoretical formula :-)....I
like to keep an open mind and am always trying new ways and things to try to
help someone get better vision :-)
Heck of a hobby.....my wife gets tired of me tinkering and even running
to the lab in the middle of the night once in awhile when I think of something
that may work and am over there tinkering away half the night :-) ...or worse
my accountant wondering how I could write off some of the lens blanks I ramble
through while playing ..:-)
In the long run Iguess it's worth it...for every 10people who have bit
my head off or yelled ranted and raved...there is always some child or elderly
person who appreciated the fact I improved something that they were lacking
before:-)....well worth the effort...even though at times its
frustrating...I'll sell anyone a Lab if they are interested..:-) I'm just about
ready to hang it up one of these days....alot of grumpy people out there and
OD's and OMD's and opticians who are always blaming the "dumb lab guy"...LOL
Oh before putting the ones to sleep that you missed Brad :-)...have a happy
holiday...
Jeff
Joe Fischer
: Hopefully here you aren't thinking this curve combination with index
:
: A 1.66 index with a nominal of -4 (well correctly stated as a +4 :-)
: and a -9.00 occular
: curve will give me a total combined power to get the -15 correction ....the
: power you are listing would be in the -18 range.....but thats comparring
: apples and oranges really..:-)
I have been wondering about the problems with
selecting grinding tools for the various different
index of refraction lenses.
As I understand it, with ordinary glass, the
tool was simply the base curve plus the prescription,
but with several different indices, is there any
math formulas or charts furnished or available?
Thanks for all the great discussions,
Regards,
Joe Fischer
Brad Rogers
> Hopefully here you aren't thinking this curve combination with index of 1.66
>and a Vrt of 12 mm is giving you a power of -15 :-)
Er Yes. -4.00 + (-11.00) = -15.00 Centre thickness is only 2mm!
>
> A 1.66 index with a nominal of -4 (well correctly stated as a +4 :-)
>and a -9.00 occular
>curve will give me a total combined power to get the -15 correction ....the
>power you are listing would be in the -18 range.....but thats comparring apples
>and oranges really..:-)
Ok THIS is what I am obviously being dumb about! - I just can't
understand it. Please could you explain in a bit more detail exactly
what powers (including +/-) are on what surfaces?
> Basically what I have seen over the years using every text I've read and
>formula from here to there do not always translate to the real "world" as well
>as to each complication.
>so even though theorectically you stated the facts I have had great success in converting alot
>of those high myops into other ways of cutting (like the bi- concave) and in
>the highest % probably in the 50% range they have said they could tell NO
>difference optically from what they had to what they have :-)
Fair enough. The most important thing is that it works!
> Oh before putting the ones to sleep that you missed Brad :-)...have a happy
>holiday...
And you Jeff
Brad
SpecOPTICS
>tool was simply the base curve plus the prescription,
>but with several different indices, is there any
>math formulas or charts furnished or available?
>
>
There are some formula's out there...I have some of them and they are about
7 miles long I don't know if you really would want them or not :-)
The main factors in figuring the occular corrective curve are:
1) Base curve being used
2) index of material
3) index of tool (cr39 or glass)
The other factors that can be manipulated are the vertex power, eliptical
error and lens thickness
you would be better off getting a calc program (RXPII+,Solacalc,OMICS
etc etc etc) and let it do the math for you :-)
I use RXPII+ and OMICS that seem to do a fairly good job...besides the
power problems
, you have slab configuration, vertex computations,prism compounding , prism
conversion and on and on.....nifty little tool :-)
If you have to have the exact formula I can find it somewhere with
basic conversion of power less base curve accounting for eliptical error all
based on a glass too; index(which the majority of the labs use)
If you want to do some digging on your own a good place to start is finding
Coburn Gerber and getting some charts from them....usually they charge an arm
and leg for any optical things though....
Jeff (spe...@aol.com)
SpecOPTICS
Brad wrote: (and is working hard at unerstanding :-)
>> Hopefully here you aren't thinking this curve combination with index of
>1.66
>>and a Vrt of 12 mm is giving you a power of -15 :-)
>
>Er Yes. -4.00 + (-11.00) = -15.00 Centre thickness is only 2mm!
>>
LOL....no the combination of power you are talking about (even I was off
some :-) actually comes to a
-4 nominal curve(True curve -3.79) in a 1.66 index at 2mm CT with a
-11.12 occular curve will give you a power of -18.50!!real precise would be a
-18.37..but whats a 12th to this power... LOL
The major difference in what you are figuring is you are not allowing
for the indice of the material you are working in..looks like you're trying to
plug in a simple equation to get the answer such as if I have point (A) and
subtract point (B) it will give me a power... on the surface you're wanting -4
+ -11 to equal -15 it's not that simple in optical equations (allowing for
"physical" material and visual acuity.) Your wanting to know how much to bend a
length of light to get the best possible corrected acuity..... because we are
dealing with a number of factors that will effect the amount of curvature
needed to get a wanted correction...even in CR39 a - 4 nominal and a -11
occular is :
-4 nominal(TC -3.79) with -11 occular with an index of 1.498 (cr39) and
a 2.0mm CT would give you a -13.87 total power..:-)
When converting in a bi-concave formula you are splitting powers
unequally as well as accounting for index of refraction.
If by chance when you figured your calcualtions and mixed of the (-)(+)
sign for the front the back curve (occular) would have been in a CR39 at 2.44
(True curve of 4.10) the back curve to get your correct power of -15 (what you
used in you post) then I would have to cut a -20.12!!!!LOL...talk about a fish
bowl!!!LOL
lens curves and index I used were:
1) -4 nominal (actual true curve -3.79)
* when you see a lens curve being written you'll see it one of two ways a
(-) concave,(+)
for convex (both dealing with front surface)
2)index of 1.66 on the first and 1.498 the second
3) Tool index for both problems were based on a glass index tool(1.530)
4) CT both at 2.00mm
5) vertex at 12mm (this is a fluid option) depending on the vertex of
the phoropter you can allow for fitting vertex in relation to refracting
vertex. (Disto-meter comes in handy here ) :-) unless your optics programs have
a vertex sub program...
In a + power flatter the occular curve higher the +...in a - power more
curvature gives you more power....all based on the nominal curve (front surface
of the lens you're grinding)
The nominal curve (front) is and can be figured by either
sign..depending on the way your curve is running...in a (+)selection its convex
in a(laymen terms.".bubbled out" :-)
( -) selection its concave....(looks like a "bowl").
That can be swapped to the back surface as well (occular)
depending on the base curve and desired power....
Then ontop of these two things the main basic components are index of
material and index of tool...the rest is fairly simple.....LOL
The main problems encountered is that when deailing with either a
convex or bi-concave instead of a strictly concave occular curve is that the
way the lens is bending light to get a correction....in a convex (eliptical
shape) the main problem is tunnel vision and induced prism...in a bi-concave
its a distorted periphral field & oblique astigmatism.....but I would rather
work in a myopic RX any day then a high hype...I HATE cutting convex lens!!LOL
Basically we are taking a desired correction and trying to come to the
same answer BUT manipulating the process by substituting indexes and nominal
curves to effect the out come...either cosmetically or visually...or hopefully
some of both :-)
Maybe someone has a simple or easier understood explanation...but if not
feel free to ask away....I like jabbering on about optics..LOL
Jeff (spe...@aol.com)
Brad Rogers
Now I understand Jeff.
What you are calling the "Curve" is actually the curve of the TOOL you
are using for your generator - and it is calibrated for glass
(n=1.530) - It is NOT the _actual_ power of the surface (which was
what I was working with). This (partly) explains the discrepancies.
Another discrepancy is illustrated by the two examples you quote;
> -4 nominal curve(True curve -3.79) in a 1.66 index at 2mm CT with a
>-11.12 occular curve will give you a power of -18.50!!real precise would be a
>-18.37..but whats a 12th to this power... LOL
No it won't Jeff! -3.87 on the front and -11.12 on the back will give
_approximately_ -14.99 (IGNORING THICKNESS). You are about 4.50D too
strong - about the same as the front curve. Is it possible you may
have miscalculated?
>.even in CR39 a - 4 nominal and a -11 occular is :
> -4 nominal(TC -3.79) with -11 occular with an index of 1.498 (cr39) and
>a 2.0mm CT would give you a -13.87 total power..:-)
That's nearer the mark.
> The major difference in what you are figuring is you are not allowing
>for the indice of the material you are working in..looks like you're trying to
>plug in a simple equation to get the answer such as if I have point (A) and
>subtract point (B) it will give me a power... on the surface you're wanting -4
>+ -11 to equal -15 it's not that simple in optical equations
Of course it is! One of the first things most optical
students/Opticians/Eye docs/ Optometrists/Ophthalmologists are taught,
i the simple approximate formula for thin lenses;
Fv'=F1+F2
. . .where F1= Front surface power, F2= back surface power, Fv'= Back
surface power
(IGNORING THICKNESS- the refractive index and thickness account for
only a small deviation from a truly accurate figure (10ths of a
dioptre at the most))
so -15= (-4.00)+(-11.00) - (within say (+/-) 0.75 D)
Anyway, lets put this one to bed Jeff - I'm putting myself to sleep!
Merry Christmas.
Brad
Specs31
Both examples are correct in power...that is the point I was making when
you are trying to figure it out with simple math equations....even though the
curves plugged into BOTH examples are almost identical to the tee...the major
factor you are leaving out is taking the index of refraction of both lens......
I can cut the eact curve on three seperate materials...and will get three
different RX's!!!LOL.......
The closer you get to the accepted natural index of 1.53 the closer you can
get to -4+-11=-15....anytime that index changes those same curves are being
manipulated into producing different refracted powers....
It all has to do with how the index of the material "warps" or "bends"
light....
Well it makes for interesting conversation for the OD's, opticians and lab
teachs stuck in the corner at a boorrrrrring christmas party...but to everyone
else I'm sure it's a real snooze fest!!!!LOL
Merry Christmas.....
Jeff
Joe Fischer
: Joe Fischer wrote:
: > As I understand it, with ordinary glass, the
: >tool was simply the base curve plus the prescription,
: >but with several different indices, is there any
: >math formulas or charts furnished or available?
:
: There are some formula's out there...I have some of them and they are
: about 7 miles long I don't know if you really would want them or not :-)
No thanks, :-), I don't expect to be making eyeglasses,
I just wondered if opticians had to guess at what tool to
use for various indices of refraction, it would be a pain
to have to try one tool, and polish, and use the lensometer
to see how close, then grind and polish some more.
And going too far and getting too thin could mean
needing to charge more for making high index lenses.
: The main factors in figuring the occular corrective curve are:
:
: 1) Base curve being used
Is the base curve an actual curve, or an actual power?
: 2) index of material
: 3) index of tool (cr39 or glass)
I don't understand, are tools made special for more
than the standard index of refraction (that could mean a
lot of tools to stock). I was assuming standard tools
would be used, but a weaker curve put on a high index
material.
: The other factors that can be manipulated are the vertex power,
: eliptical error and lens thickness
I saw you mentioned lens thickness before, isn't it
the difference in index of refraction of the materials on
both sides of the surface that determines how much light
is bent?
: you would be better off getting a calc program (RXPII+,
: Solacalc,OMICS etc etc etc) and let it do the math for you :-)
Thanks for responding, I am glad to hear there
are computer programs, I never even though of that. :-)
Regards,
Joe Fischer
SpecOPTICS
>than the standard index of refraction (that could mean a
>lot of tools to stock). I was assuming standard tools
>would be used, but a weaker curve put on a high index
>material
you can have two set of tools..one based on index of glass or the index of
cr39....
The person doing the work will compensate for the index of the tool
set...you usually keep a set of tools that run in the range of 3.00 to 10. with
increaments of .25 ..the tool would come in either sphere or compound...giving
an example starting with a 3 tool the next tool would be 3.00/3.25,3.00/3.50
and on and on.....a full set will run around $6500 or higher depending on the
range you want to go....I have them from minus to plus tools...oh around 2800
tools (give or take) and the run around $5 a pc. if you order them pre-cut...I
have a machine to cut my own tools if needed though...might as well throw in
another $8 to $10thousand if you want that!!LOL
You only have one set of tools and what the mathmatical part is that as
you go through the different indexes of lens you change curves of the tools to
compensate.....
A simplified version just to make it understandable...say you walked
in and needed a +2 sphere lens.....I would pull a pair of CR39 lens and grind
it and run it on a +4 tool ( 6 base (front curve)- 4 =(+) 2 sphere ...thats
kinda simplified but you get the general idea....maybe better to undestand..I
have a lens that if I take it and make the back side plano(flat) and check that
lens in a lensometer then a 6 base lens with a plano back would give me a power
of +6 sphere...now to get that +2 sphere power I would take that 6 base and
put a curve of +4 on the back and that would give me the power wanted ,,,to
clarify, anytime you see in these posts people talking about plus curve its
curves that bubbles "away" from your eye ...
Taking that same lens and now you decided you wanted it on a high
index material....that same lens now would be run on a different back curve
(tool) but you would still end up with the wanted power !! :-)
BTW tools also come in three materials...alloy/steel, plastic or
foam....first type last forever (or seems so :-) second wears out fairly fast
comparring to the first...and the last (foam) is a one run and toss it in the
garbage..LOL
>I saw you mentioned lens thickness before, isn't it
>the difference in index of refraction of the materials on
>both sides of the surface that determines how much light
>is bent?
The index of the material is has to do with the whole lens not just
"sides"....as for thickness...if a lens is left to thick it will distort as
well as if it is taken to thin...it will effect the power either through
"warping" or if thick it can be picking up defects in the lens when it was
molded
Eaisest thing to do is go to a local lab or eyeglass place that has a lab
and watch them do a pair of lens :-)...once you see it you get a better
understanding about the mathmatics pushing the process....
happy holidays!!!
Jeff(spe...@aol.com)
Robert Shanbaum
convention (in North America, anyway) is to refer to lens and tool curves in
"shop index" which is 1.53, which means that, even given a "true curve",
you need to know the index of refraction of the material in order to figure
out the actual surface power of either lens or lap.
A 1.66 lens with a "true curve" of -3.79 would have a surface power of
(660/530) * -3.79 = -4.72. Using the thin lens approximation, you'd want a
back curve of -11.27 to achieve a back vertex power of -15.00. The 1.53
tool used would be a 9.00 (if the lab's tools are rounded to the nearest
0.12D, which is common; though rounding to 0.10D is becoming more popular)
as 11.27 *(530/660) = 9.05.
This convention has been, and continues to be, a source of confusion amongst
even experienced opticians. In continental Europe, the practice is to refer
to lens curves in the index of the material of which the lens is made, and
to grind them on tools likewise labelled. This does, however, lead to a
proliferation of grinding tools.
Brad Rogers wrote in message <367d2f84...@news.globalnet.co.uk>...
Joe Fischer
: This convention has been, and continues to be, a source of confusion amongst
: even experienced opticians. In continental Europe, the practice is to refer
: to lens curves in the index of the material of which the lens is made, and
: to grind them on tools likewise labelled.
:
: This does, however, lead to a proliferation of grinding tools.
That statement wins the "Understatement" prize, :-)
I do have an interest in optics, but have never
made eyeglasses, although I have watched.
Can anyone tell me if grinding pads are made
with the cylinder built in, in order to avoid such a
large number of tools?
Joe Fischer
Specs31
>with the cylinder built in, in order to avoid such a
>large number of tools?
>
>
No...you have to choose the correct tool to get the desired power....tools
come in sphere's as well as compound curves so you can get the cylindrical
correction you need...
Give an example of how a tool would be labeled is ..I want a 0 sph. .75 cyl.
x100 on a lens with a base curve of 6.25( TC6.20) Then the tool I would run it
on would be labeled 6.25/7.00...this tool would give me a power of Plano with a
.75 cyl.
Atleast this way we only have to keep a set of tools numbering in the
1,200's or so (depending on your range) BUT if you wanted a pad with the
compound in it you would have to keep rolls of every possible power combination
out there!!LOL...I wouldn't have any room in the lab for anything else!!LOL
Good guess though..just not cost effective or practical...
Jeff
Specs31
>convention (in North America, anyway) is to refer to lens and tool curves in
>"shop index" which is 1.53, which means that, even given a "true curve",
>you need to know the index of
Robert...
Actually somewhere in that group in this thread ?I did say all that...just
not all at once :-)
As the thread stretched out I was just answering question as they were
posted and answering it in a "strung" together set of posts :-)
Add another lump of coal to the list :-)
Jeff
Robert Shanbaum
think it was a British company (J.S. Wylde?). I think it's a neat idea.
Joe Fischer wrote in message <368d2...@news.iglou.com>...
>Robert Shanbaum (shan...@email.msn.com) wrote:
>: This convention has been, and continues to be, a source of confusion
amongst
>: even experienced opticians. In continental Europe, the practice is to
refer
>: to lens curves in the index of the material of which the lens is made,
and
>: to grind them on tools likewise labelled.
>:
>: This does, however, lead to a proliferation of grinding tools.
>
> That statement wins the "Understatement" prize, :-)
>
> I do have an interest in optics, but have never
>made eyeglasses, although I have watched.
> Can anyone tell me if grinding pads are made
>with the cylinder built in, in order to avoid such a
>large number of tools?
>
>Joe Fischer
Joe Fischer
: Give an example of how a tool would be labeled is ..I want a 0 sph. .75 cyl.
: x100 on a lens with a base curve of 6.25( TC6.20) Then the tool I would run it
: on would be labeled 6.25/7.00...this tool would give me a power of Plano with a
: .75 cyl.
:
: Atleast this way we only have to keep a set of tools numbering in the
: 1,200's or so (depending on your range) BUT if you wanted a pad with the
: compound in it you would have to keep rolls of every possible power combination
: out there!!LOL...I wouldn't have any room in the lab for anything else!!LOL
I meant have the tools with the sphere, and the
cylinder in the grinding pad. t would require some
way to index and hold position for grinding bifocals,
but I guess if you have the compound tools already,
there wouldn't be much of a market for a new system.
Regards,
Joe Fischer
Specs31
low end production would it seem workable to me....
Time is money...so you would have to keep multiple rolls of all three pads
(first,second&polish) and also in glass pads as well as poly...then you would
need all the combinations of cylinder or atleast a large enough range to make
it cost effective...say upto a -3 cylinder...then you would have to take the
time to make sure each pad is alligned correctly in all three phases.....if not
you'll end up with a ton of swirls and axis problems :-)
The only time it would be a pain was if it was some odd ball job where it
was past the average cut or a crazy prism job or a slab..etc. etc. etc. and you
need to use the cylinder machine as a tool to help get a end result.....
In say a small optical shop (20 or 30 jobs a day) this maybe a workable
idea...mainly save room from having to stock a bunch of tools ...as well as the
cost of the tools,,and all the problems that come along with that..so you might
as well throw in a tool cutter :-)
If someone comes up with a good plan I would be up for it..I could sell
off about 1000 or so tools and dump my two tool cutters!! :-)
Jeff