Re: ATM Star Test

[Jim Fitch <jf@arn.net>]

Tom,

Peter pretty much said it all. Plastic bifocals have a ledge where the
stronger curve provides the bifocal power. Glass lenses use two types of
glass with different index of refraction to do the same thing. The
exception is an executive glass bifocal which does have a ledge. 

The front curve of the lens can be flatter or steeper and is usually
chosen to reduce aberrations, but also influences the thickness of the
lens. So high minus power lenses generally have flatter front curves to
keep the edges reasonably thin even though there may be more
aberrations.

High index glass can be thinner and have less chromatic aberration but
plastic is lighter. 

I'm not going to TSP this year, but am planning to go to Okie-Tex. Are
you going ot both?


> 
> > German glasses were lenses of glass instead of plastic.

> A piece of higher refractive index is fused in place for the lower element
> which needs more lens 'power'.
>