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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