In article <B42FBB2596683DD...@g3.durrant.co.uk>, p...@durrant.co.uk (Paul

Having been through this previously myself here are a few abbe values for
you.  I previously gathered these from posts to this board and the web.

                  Index       Abbe
CR39              1.499       58
Sola Spectralite  1.537       47
Seiko             1.56        42
Seiko             1.60        37
Pentax            1.6         36
Optima Hyperindex 1.66        32
Poly Carb         1.586       30

My correction is almost -10 with minor astigmatism in both eyes. I
purchased frames with smaller lenses, the sola spectralite with a 1mm
center, AR coating, edges polished.  Considering my correction the edges
are fairly thin, the glasses are light and they look fairly decent.  The
optics are quite good.  Prior to buying these glasses I used to buy
glasses from Lenscrafters, Vision World and that ilk (in high index they
only sell polycarb).  I had totally given up on reading at night as my
eyes simply got to sore.  That's all better now.

I just emailed Sola to find out what the abbe was on their new ViZio high
index (1.66) aspheric lens are. From their literature you'd think this was
the "second coming" of lens technology putting it head and shoulders above
the competition. Well here are the numbers:

                Index    Abbe
Vizio plastic    1.66     32
Vizio poly       ?        30

These really are not spectacular numbers. Am I missing something here? In
looking at their info on the net, it shows them comparing the maximum area
of sharpness on ViZio verses another brand poly lens and it walks all over
it. Doesn't abbe value predict overall area or viewable sharpness and
chromatic aberrations?

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In article <dcmiller1-1710991901180...@jdsl236.mpls.uswest.net>,

In article <380b964...@bonaparte.pixi.com>,

I suspect that they're talking about monochromatic sharpness - i.e. only
comparing the shape of the lens.

A lower Abbe value will, of course, make anything off the optical centre
less sharp, given otherwise similar lenses.

Paul

   Your stuck on the abbe problem, the major distortions in a visual field of
the lens is NOT due to an abbe problem but oblique astigmatism :-) The
diffrence your seeing in all the lens is NOT that its an aspheric cut its an
ATORIC cut :-) thats what is helping eliminate , well thats a bad word it
neutrilizes the effects of an oblique astigmatism.

    Optics are a little more complex then just considering "abbe" value, sadly
the majority of opticians out there do not seem to understand what exactly the
problems are and the "how & why" we correct the problems that come with using
non standard indexes.

JeffT

 Actually after reading the posts under this thread some of the posters (as
well as yourself:-) is mixing what exactly abbe value and oblique astigmatism
is in a lens.

   You seem to understand what chromatic abberition is but the "distortion"
everyone is mentioning is more due to oblique astigmatism NOT because of the
abbe value of the lens. The lens being dicussed i.e. the ViZio and the charts
you guys were talking about was showing what an oblique astigmatism effects the
visual field in a lens, how you try to neutrilize this effect is by
manipulating the type of front surface of the lens , in the ViZio case it with
an ATORIC design while the Spectralite and most of the other higher indexed
lens do it with the use of an aspherical surface compared to a spherical.

    The thing to remember is the lens is NEVER going to be any better the the
dispenser will let it. :-) with out the proper fitting procedure and frame
selection you would be defeating the purpose of using an advanced design and
index. If you keep the frame as small as possible with a comfortable visual
field and keep the vertex 10 to 12 mm with the proper panto tilt in the frame
any of the lens do a fairly good job.
    With your RX , any of the 1.55 to 1.60 range would be more then effective
(making sure to get an AR coating)
    In fact I just put out a pair of Super16 from Seiko and have them right
here in my hands R-2.25 sph. L-3.00sph. in a Rembrand masterpiece frame 50x18
eye the edge thickness is 2.64 mm in the right 2.98 mm in the L (I guess you
can tell I have digital calipers:-) CT is 1.2 OU
    But the chromatic effect is not noticable in the lens at all no matter
which angle I turn them , the AR coat is great about letting light travel
"through" the lens and not reflect everywhere.

    With PROPER fitting and design selection and frame any of the lens would
work with your rx, the major problems you usually notice is reflections and
edge reflections not as much as the internal chromatic effects.
    Just go with any of the aspherical or atoric designs and make SURE they are
fit properly and you should have no problem.
 AND for goodness sakes invest in a good AR coating :-) With your RX I would
probably recomend a Spectralite if you walked into my store or called me if you
were a wholesale account. BUT if it is fit incorrectly even though your RX is
not really high you will notice a difference :-)

JeffT

In article <19991019191028.15535.00000...@ng-ce1.aol.com>,

Yes, but the original complaint was about chromatic abberation. I could
well be wrong, being just a user not a maker, but I thought the only thing
that would reduce the chromatic abberation was a higher abbe value?

Paul

  Actually he was confusing the terms "chromatric" and oblique astigmastism.
  He mentioned the "sweet spot" and that is where the optical center is and as
you move across the plane of the lens the natural effect is you get induced
astigmatism at an oblique angle ..non technical...it looks "blurry" or
distorted :-)
    Chromatic abberation is that "rainbow" you see out in the periphrel of the
lens..the lower the abbe number the higher the degree the lens breaks light
down into natural lengths ..result, you see colored lines beside images. :-)
and that has the effect of "feels blurry kinda" but its just the lens breaking
the ambient light down into its natural wave lengths.
      The whole idea as you get higher in index you try to use the high tech
surface designs to offset the problems that you get by using a higher
refractive index. (aspherical/atoric) also you try to make sure you get proper
fitting from a good optician that understands optics :-)

   JeffT (spec...@aol.com)
 "optician but prefers the lab work" :-)

In article <19991020130254.25944.00000...@ng-cn1.aol.com>,

Having re-read it, I thought he wqs complaining about both, but I could
have mis-read it.

I may have mis-interpreted you here. Are you saying that induced
astigmatism is a function of the refractive index of the material? i.e.
that (other things being equal) glasses made from a 1.5 material will have
less induced astigmatism than glasses made from a 1.66 material?

Again, are you saying that aspherical designs are intended to reduce
induced astigmatism, and if so, would their use with a low index material
result in an even better result than in a high index material?

Intrigued, but confused :-)

Paul

  >Again, are you saying that aspherical designs are intended to reduce

     Yes, the index of the lens does increase the amount of astigmatism at an
oblique angle so what we do is use a aspherical design or atoric to help
neutralize this effect the surface design offsets the amount of induced
astigmatism. It's still there but it get "pushed out even further to the edge.
Thats why you always see opticians or OD's telling you to get a small as frame
as possible (that still looks cosmetically good) that outer edges are just a
huge distraction and even if it were there it would not be of any use to you.

    Astigmatism at an oblique angle is just one of the give and takes you have
when fiddling with the index and using diffrent compounds of material, the way
you get the diffrent refractive indexes is done by making the lens in different
materials. The way these materials effect "light" and transmission is where you
have the problems, besides getting chromatic problems and oblique problems
internal reflections and loss of light transmission increase as you go up in
index :-) you lose 10% of light tranmission between a cr39 to a 1.60 index
lens(frenels equation)!! :-) Thats the reason you want to make sure you always
get a anti reflective coating as you increase index. Of course the majority of
opticians don't seem to know this is the reason :-)

    The majority of indexes are available in both a spherical and aspherical
choice :-) Sorry to say that most opticians don't seem to grasp the physics
behind optics so don't undertand the "hows" and "whys" ..most don't even seem
to know whats available out here in the market, specially the "chain" store
types who are limited in the choices they have to actually dispense.

    We had a thread a few weeks back (I don't remember the name of it I think
it was headed undedr "atoric curves" or someting like that) where I went into
very indepth explanation of aspherical/spherical/atoric curves.. I think it was
Mike Tyner that I was talking to ask him maybe he remembers the thread. Do a
search on DEJA news it will explain alot of what we are talking about when
talking on curve design and high indexes and how it effects light.

      People want "thinner" "lighter" material  thats possible but we still
have to deal with the laws of physics, when you gain in one area usually you
lose in another but then you try to offset the internal problems that arise
(chromatisism,oblique powers,loss transmission) by using diffrent types of
surface design and coatings. All these things can be improved upon but combined
you still need to have an experinced dispenser who understands optics and frame
selection and fitting to help even further in the process. I can give you a
perfect set of lens and the OD's refraction is dead on but if the optician
fails then its a waste of time :-) All three sectors need to fit together to
get the best results.

     Basically you have a degree of "clear" visual area in a lens, as you leave
that area  there is not really much that can be done to enhance it your eye
rotates BUT the lens is sitting still so as you move across the lens from the
OC (optical center) across the plane of the lens the "power" changes as you
look through the lens at an angle (tada your "oblique" explanation :-) Take a
sheet of glass or, since you wear glasses hold them out in front look straight
through the lens then start tilting it ..see how things "bend" :-) thats the
same thing we are talking about. Of course thats the "extreme" case but it
gives you a good idea of how it works. The higher the power the less area you
have before starting into those oblique problems. Hence the higher indexes, the
advantage of high index is not really the wght. BUT you cut "less" curve to get
the same corrective power. there by less "fish" bowl so you have a wider area
before your lens starts to "warp" images. :-) that in conjunction with the
aspherical or atoric design helps neutilize it even into a wider field :-)

  I could ramble on all day about optics but I gotta get back to work :-) Let
me know if I missed any spots or if you havn't found that thread on curves.

JeffT

In article <19991021150313.20345.00000...@ng-fb1.aol.com>,

Thanks, I'll go and look it up.

Thanks for your posts. So far I've gathered that if I want the widest
undistorted field of view, I should

(i) Get a good optician
(ii) Get small frames that fit close to the eye
(iii) Get CR-39 spherical lenses

My prescription is around -6 and -2 astigmatism.

Because I work with computer screens all say, i find chromatic abberation
really irritating - I keep on wondering if it's the scren or my glasses!
Slight blurring off-centre isn't so troublesome - it's only when the colour
split that I really notice.

Paul

You hit two of three! Yeah!! a Cr-39 spherical (actually a corrected curve)
would do nicely and save money over a true aspheric. Add an AR coat to round
it off if you have lights behind you that can't be moved and show up as a
ghost image.  Please make sure you are using correct screen placement and
distance guidelines. A screen located too high will result in reduced
blinking rates leading to dry eye sensations and headaches!

Carl

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