Mike Williams <nos### [at] econymdemoncouk> wrote:
> "Researchers have now
> shown, through rigorous mathematical analysis, that a perfect lens is
> not possible, said Kevin J. Webb, a professor of electrical and computer
> engineering at Purdue."

  That's a rather surprising result (or not, after you study optics for
some time, I suppose).

  Seems that the best possible lenses available with current technology
are the so-called apochromats which try to minimize both spherical and
chromatic aberrations (but are not *perfect* either), and are really
expensive: http://en.wikipedia.org/wiki/Apochromat

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
> Stephen <mcavoys_AT_aolDOT.com> wrote:
> > Is this the sort of thing?
> > http://www.reference.com/browse/wiki/Lens_(optics)
>
>   Why not give an url to the original article, which has the images etc.
> intact and not raped like in that site?
>

You really take the biscuit, Warp. I just hope you are not as rude and
pretentious to your students as you are to people on these newsgroups.

Stephen

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Stephen <mcavoys_AT_aolDOT.com> wrote:
> >   Why not give an url to the original article, which has the images etc.
> > intact and not raped like in that site?

> You really take the biscuit, Warp. I just hope you are not as rude and
> pretentious to your students as you are to people on these newsgroups.

  Exactly which part of my sentence was rude?

  And I don't have any students. (Not that it matters.)

-- 
                                                          - Warp

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> > > Is this the sort of thing?
> > > http://www.reference.com/browse/wiki/Lens_(optics)
> >
> >   Why not give an url to the original article, which has the images
> >   etc. intact and not raped like in that site?

> You really take the biscuit, Warp. I just hope you are not as rude and
> pretentious to your students as you are to people on these newsgroups.
I struggle to find a way of reading Warp's comment to be more rude than
what sites like the one you link to do to Wikipedia. Now, as a supporter
of truly free information, I must accept that there is nothing *wrong*
with what they do, but it's damned ungentlemanly, and it isn't good for
promulgation of information.

-- 
And for mile after mile you'll never see me tire/You'll never me me slow
down for a while/'Cause I am the fox, like it or not/I'm always gonna be
there  running over the rock/ Yes I am the fox,  a fascinating cross/ Of
sharp as a whip and tough as an ox              Bernie Taupin, 'The Fox'

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Warp <war### [at] tagpovrayorg> wrote:
>   Exactly which part of my sentence was rude?

perhaps he is referring to the raping you said the website commited against
the original wikipedia article, which it indeed did:  the wikipedia article
is far more complete and with plenty of illustrative images.  i don't have a
problem with the sentence and would have used it myself to describe the
situation, but i guess some people feel it a bit agressive, even though the
real agression is against Wikipedia...

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Mike Williams <nos### [at] econymdemoncouk> wrote:
> Wasn't it Stephen who wrote:
> >Warp <war### [at] tagpovrayorg> wrote:
> >> G?nther Dietrich <gue### [at] despammedcom> wrote:
> >> > Have you already tried the intersection of two CSG-speres? That's just
> >> > what real spheric optical lenses are.

One of the first renders I ever did in DKBtrace was such a shape, but I did
it in wood, not glass, on a 30MHz Amiga. :)

> The lenses in eyeglasses tend to be more complicated than that. Most
> eyeglass prescriptions have a spherical and a cylindrical component, and
> some (like those for my mother who has one eye that doesn't point in
> quite the right direction due to a mild stroke) have a prism component.

A lens with spherical & cylindrical components is called a toric lens,
because it can be thought of as a section from the surface of a torus.

Many people require a toric prescription to correct for astigmatism, but I
wouldn't say it was a majority. I take orders for contact lenses for one of
the world-leading manufacturers, and we sell a lot of toric lenses
(especially after recent improvements in the technology), but most of the
orders are spheric lenses to correct for short-sightedness. Of course, the
story for spectacles could be different, but severe astigmatism is pretty
rare.

Why do spheric lenses work? Because sin(x) ~= x for small x.

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Warp <war### [at] tagpovrayorg> wrote:
> Stephen <mcavoys_AT_aolDOT.com> wrote:
> > >   Why not give an url to the original article, which has the images etc.
> > > intact and not raped like in that site?
>
> > You really take the biscuit, Warp. I just hope you are not as rude and
> > pretentious to your students as you are to people on these newsgroups.
>
>   Exactly which part of my sentence was rude?
>
>   And I don't have any students. (Not that it matters.)
>
> --
>                                                           - Warp

Dear Warp,
I thought that you were being deliberately rude. I see that you were not and
I apologise. I jumped to a wrong conclusion. The URL I posted was not the
one that I meant to post or thought I had posted.
Since you ask what I part of your sentence I found rude I will tell you
privately via your tag email if you still want to know.

Regretfully

 Stephen

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Stephen <mcavoys_AT_aolDOT.com> wrote:
> I thought that you were being deliberately rude. I see that you were not and
> I apologise. I jumped to a wrong conclusion.

  My style of writing in the internet is often quite direct. This often
makes me sound rude even though it's not at all my intention.

  At one point, many years ago, I simply got tired of overusing smileys
everywhere and trying to think of "soft" ways of writing my opinions in
order to try to avoid anyone get offended. I thought something along the
lines of "heck, why am I going through all this trouble to try to avoid
someone getting offended by something I am not trying to say in an
offensive way?"

  Granted, it's not a very smart way to behave, but after interacting in
the internet so actively for so long as I have, you just get tired at
some point and stop caring. I just say what is in my mind.

-- 
                                                          - Warp

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PM 2Ring wrote:
> Many people require a toric prescription to correct for astigmatism, but I
> wouldn't say it was a majority. I take orders for contact lenses for one of
> the world-leading manufacturers, and we sell a lot of toric lenses
> (especially after recent improvements in the technology), but most of the
> orders are spheric lenses to correct for short-sightedness. Of course, the
> story for spectacles could be different, but severe astigmatism is pretty
> rare.

I don't know how rare it is, but I am one who needs astigmatism 
correction as well as for myopia (near-sightedness).  But a few years 
back, I dropped my glasses (standard glasses, not contacts) and one of 
the lenses cracked, so I went to my optomitrist for a replacement.  Took 
about a week to have it made.  In that time I was still using this 
cracked lens -- a minor annoyance but no big deal.  When I got the 
replacement lens, my first look through it seemed very odd.  The doctor 
re-checked the prescription and said the yes, it was the right 
prescription -- but rotated 90 degrees!  I had to use that offset lens 
for the next week while a proper replacement could be made.  Boy! what a 
strange sensation -- when I walked it felt like I was leaning over at an 
angle.  :-)  All turned out okay eventually, of course, but it was sure 
a strange feeling during that time.

      -=- Larry -=-

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Larry Hudson <org### [at] yahoocom> wrote:
> PM 2Ring wrote:
> > Many people require a toric prescription to correct for astigmatism, but I
> > wouldn't say it was a majority. I take orders for contact lenses for one of
> > the world-leading manufacturers, and we sell a lot of toric lenses
> > (especially after recent improvements in the technology), but most of the
> > orders are spheric lenses to correct for short-sightedness. Of course, the
> > story for spectacles could be different, but severe astigmatism is pretty
> > rare.
>
> I don't know how rare it is, but I am one who needs astigmatism
> correction as well as for myopia (near-sightedness).

Same here. Astigmatism seems more common with those of us of European origin
than with Chinese people, who often have severe myopia.

> But a few years
> back, I dropped my glasses (standard glasses, not contacts) and one of
> the lenses cracked, so I went to my optomitrist for a replacement.  Took
> about a week to have it made.  In that time I was still using this
> cracked lens -- a minor annoyance but no big deal.  When I got the
> replacement lens, my first look through it seemed very odd.  The doctor
> re-checked the prescription and said the yes, it was the right
> prescription -- but rotated 90 degrees!

Yes, that can happen sometimes... usually mistakes like that are picked up
before the lens gets to the patient, but not always.

> I had to use that offset lens
> for the next week while a proper replacement could be made.  Boy! what a
> strange sensation -- when I walked it felt like I was leaning over at an
> angle.  :-)  All turned out okay eventually, of course, but it was sure
> a strange feeling during that time.

I bet! If that happened to me, I'd get motion sickness. :)

FWIW, I've taken quite a few toric lens orders today.

<Lame attempt to get back on topic>
I just had an idea: I could use POV to illustrate what the world looks like
through my eye, by fitting the POV camera with a lens which is the inverse
to my prescription.

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