On Mar 16, 1:53 am, Robert Martellaro <rob...@nospam.com> wrote:
> On 15 Mar 2007 04:03:21 -0700, "carrera d'olbani" <dolb...@yahoo.com> wrote:
>
>
>
> >Here is the article from Wikipedia. It says that lenses from the
> >materials with the higher refractive index have the higher chromatic
> >aberrations.
>
> It says the higher the IR the lower the Abbe value. The lower the Abbe value the
> greater the dispersion hence greater transverse chromatic aberration (TCA). This
> is the chromatic aberration created by the prismatic effect at the periphery of
> a lens when looking off axis. The formula for TCA is cF/V, where c is the
> distance from the OC (optical center) in centimeters, F is lens power and V is
> the Abbe value. According to Torgersen, acuity is not affected until TCA reaches
> 0.16 prism diopters and then it drops by one line on the Snellen chart.
>
> >The higher chromatic aberrations are perceived by people
> >as a higher "blurryness". Perhaps this is what I was witnessing. The
> >materials with the higher dispesion are said to have the higher Abbe
> >number (calculated according to the formula). The higher the number
> >the less is the dispesion, therefore, the better is the optical
> >quality. In the link below, it is stated that the Abbe number of
> >plastic (CR 39) is 59.3, which is on par with the low (1.53)-
> >refractive index crown glass (58.5). It is much higher than the Abbe
> >number of a high-refractive index glass of a 1.7 refractive index,
> >namely 36 for Hoya glass.
> >This should mean that the polycarb should have a very good image
> >quality from the point of view of chromatic aberration -- the same as
> >that of glass, in fact. But you and others say that polycarb is bad.
> >How come ?
>
> See the above.
>
> >http://en.wikipedia.org/wiki/Correct...8CR-39.E2.84.A...
>
> Lot's of good information although there are a few errors and misinformation.
> For instance, we do not use bi-convex lenses to correct presbyopia due to
> unacceptable levels of oblique astigmatism and power error. With rare exception,
> ophthalmic lenses use best form meniscus shaped lenses.
>
> Back to your original question- I still haven't heard a reason why you should
> change your lens material from crown glass, although many optical companies
> (primarily the chains) will not supply glass lenses. Most use polycarbonate due
> to profitability and reduce processing time.
OK, here is the full story. I went to the optometrist for new
spectacles (in Oz). He offered me the polycarb lenses, and I said I
did not want them, because they produced too much blur in the
peripheral vision, and thus I wanted glass. He recommended me the
glass of a 1.7 refractive index because it was lighter than the
standard glass (1.53 crown). I did not know why the 1.7 glass should
be optically worse or how much it would be worse, so I reluctantly
agreed.
When I came home, I did research on the Internet (and found the above-
mentioned article on Wiki). It said that the 1.7 glass had the
dispersion as bad as the one of the polycarb. The 1.53 crown glass had
much lesser dispersion (its Abbe number was ~60 vs. ~30 of the
polycarb/1.7 glass). I became unhappy about my future glasses :-( .
When I came to pick up the ready glasses, I saw that my new glasses
produced as blurry image on periphery as the polycarb-fitted glasses
did. When I moved my gaze 1 cm away from the lens centre towards the
periphery (what, 20 degrees ?), the image became very blurry. It was
totally inacceptable to me. I said to the optometrist about it. I
showed him the print-out from Wiki with the data on Abbe numbers of
1.53 and 1.7 glass and of polycarb.
The optometrist told me that he wanted to keep me happy, and I did not
have to buy the glasses. I asked him to fit the ordinary crown glass
lenses into the frame (to replace the 1.7 glass), and said I would pay
for the old and new lenses. The old chap said he already offered the
best spectacles for me, and he would not be doing anything more for
me. He suggested I take my prescription and go to another
optometrist.
I could not believe my ears. I decided that the old guy was not in the
business for money any more (he was practising in this prime downtown
place for 50 years); he was doing the business just for job
satisfaction. I decided that I wanted to try these low-Abbe number
lenses anyway, so I bought the glasses. As an afterthought, the
optometrist told me that I could bring the glasses for refund within 2
weeks if I find them unsuitable.
I now have these glasses. I calculated the transverse chromatic
abberation (TCA) according to your formula above, and actually found
that the Torgersen criterion was correct (the TCA was around 0.33 D
and the acuity actually dropped by about 2 lines). Interestingly, my
old glasses with the 1.53 crown glass lenses also had an acuity drop
at the same peripheral angle as the new glasses, however the drop was
not as much (maybe 0.5-1 line ?). Heck, today I was watching for the
babes walking past me, and I had to turn my head to see them better
(thus, it became evident to my companion that I was actually watching
them) (with my old glasses, I did not have to turn my head and still
saw them clearly at the periphery of the lens !).
Also, the 1.7 glass lenses give a purple or green reflection
(dependent on the viewing angle). This tells me that the lenses have
an anti-reflection (or any other) coating. The optometrist told me
that the lenses had no coating just like I requested. Is there a
coating on my lenses ?
Thanks for your attention.
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