on 6/23/02 12:58 AM, Brian Swale at bj@xxxxxxxxxxxxxxx wrote:
> 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
Brian has it right... I've read that one aspherical element replaces two in
a 'normal' design. More and more new designs are using multiple aspherical
elements, I've seen descriptions for cameras claiming four aspherical
elements... At one time the way to finish the aspherical elements was grind
and polish, which took a lot of time and drove the cost up, but advances in
glass-molding techniques now allow most consumer-grade aspheric elements to
be molded to nearly the finished configuration, so that final polishing is
minimal or even unnecessary. That's why many of the zoom point-and-shoot
cameras and digicams have aspheric elements. I suspect Olympus would use
aspheric elements in any new lens designs; I believe the IS-3 and IS-30 have
them, and probably the IS-5/50 and the many Olympus digicams.
--
Jim Brokaw
OM-1's, -2's, -4's, (no -3's yet) and no OM-oney...
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