On 23 Nov 00 at 10:11, Hugo G�vert wrote:
> On Wed, 22 Nov 2000, Shough, Dean wrote:
>
> > > > What are DO-elements?
> > >
> >
> > Diffractive Optical Elements.
> >
> > They may look like Fresnel lenses, but optically they are quite different.
>
> No, and yes. True diffractive optical elements do not look anything like
> Fresnel lenses!! That's the problem! The Canon DO elements (or the
> schematic pictures of them) looked very much like Fresnel elements.
The schematics do, but only confuse people, as that would be a
refractive lens....diffractive means 'bending light around physical edges',
and the edges in this case are the micro-gratings on an otherwise
normal-looking lens....this optical element was shown separately at
the PhotoKina....the micro-grating was very difficult to see, being
so small.
The context of the weight- and size-advantage is also interesting:
short lenses like this *can* be build, but the abberations would be
massive, impossible to correct....while DO allows a decrease in length,
without compromising optical quality....
So it's not correct to state that DO allows a short lens as such (or
that DO automatically means 'short') only that it allows a short lens
without affecting image quality....and my guess is that *given* the
choice for a short lens, a weight-advantage is inherent (but perhaps
there would still be an weight advantage when keeping the lenght the
same....but that would apply more to refractive Fresnel than
diffractive grating).
--
Bye,
Willem-Jan Markerink
The desire to understand
is sometimes far less intelligent than
the inability to understand
<[EMAIL PROTECTED]>
[note: 'a-one' & 'en-el'!]
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