Hi Rob,
I understand it to be as follows.
Usually the curved surfaces of lenses are a computed curve that is (part of) a
perfect spherical surface - as in a perfect marble or what-have-you.
The reason for using a perfectly spherical surface for all non-flat surfaces is
that they are *much* easier and cheaper to machine. But this restriction
brings a cost in less-good performance.
Aspherical lenses, as I understand it, are non-flat surfaces that have a curve
to them that is NOT part of a perfectly spherical surface. Thus, aberrations of
various kinds can more easily be computed out of the lens overall formula,
BUT the cost is in the machining - grinding, polishing etc. This may be why
some aspherical lens elements are plastic - they can be moulded, thus
reducing the cost of machining the element. The clever machining is done
but once - to the mould.
If I have this wrong, folks - I'm sure you will tell me <g>
Brian
> > > reason to use an aspherical element was to *reduce*
> > > the number of elements
> > > in a design.
> > >
>
> Could any one help with the explanation of what aspherical element is.
> Many thanks inadvance,
> Rob
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