Re: ATM Off axis Parabolas

["Andrew Schott" <aschott@cfl.rr.com>]

What about the new plastic lenses?  Aren't they all shot in a mould like
model car parts then polished?  I know some of the camera companies are
pre-casting their plastic lenses to curve then polishing to figure.  Since
eyeglasses aren't as precise, I thought they might be casting them and
serving them up that way.

Anyone who's ever worn eyeglasses for any time at all usually begins to
notice how sloppy the lenses are figured.  Like you say, they just calculate
a curve and grind it into a surface with little or no final figuring.  I've
had glasses that were so poorly figured you could see objects bend and wave
as you turned your head!  And to think that we're using these low grade
lenses to evaluate and critique optical systems 100X higher quality!  How
can someone with 20/400 vision and 2 diopters of astigmatism look at a star
through a scope and determine it to be better than 1/10th wave quality?!?  I
do it all the time>8-)


----- Original Message -----
From: "Steve Fejes" <jsfejes@swva.net>
To: <omegatroid@hotmail.com>; <ATM@shore.net>
Sent: Saturday, December 29, 2001 10:53 PM
Subject: Re: ATM Off axis Parabolas


>
> Mutalib,
>
>   I worked for many years at an eyeglass labratory where I ground and
> polished many lenses.  They have tools with astigmatism ground into them
> which are generated on a lathe.  They use sand paper or metal pads over
> these tools.  They polish with felt pads over these tools.  The lenses
> are rough ground using a curve generator which is something like a lathe
> with a spinning diamond wheel.
>
>   The tolerances are very loose, not what ATMs would call precision.
> The tolerances that we worked with were plus or minus 0.12 diopters.  A
> diopter is the inverse of the focal length.  So, 1 diopter is 1/1000mm
> focal length, 2 diopters is 1/500mm focal length.
>
>   If you could get some cheap lenses to play with it might be
> interesting, but I would not spend a lot of money thinking you are
> getting a precision lens.
>
> Steve
>
>