Hello Brian,
Thank you for your explanations. It is a word I come across often, but
never sure of its meaning.
Regards,
Rob
----- Original Message -----
From: "Brian Swale" <bj@xxxxxxxxxxxxxxx>
To: <nizora@xxxxxxxxxxxxxxxx>; <olympus@xxxxxxxxxxxxxxx>
Sent: Sunday, June 23, 2002 5:58 PM
Subject: ( OM ) (was) Re: [OM] Zuiko 200mm f/4; now aspherical lens
> 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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