On Thu, 23 Nov 2000, 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. Of
Ok, I must answer myself... I just saw the page... while I remembered
correctly about the conceptual diagram, I can not really see the photo of
the actuall element at all (my work computer has only 256 colors and all
the photos are very bad) - but it seems that it does _not_ look like
Fresnel-lens after all.
> > I expect that the big problem with the 400 mm DOE lens will be scattering
> > of light into the wrong diffraction orders. I will be very interested in
> > seeing how well this lens performs when there is a bright light source in or
> > nearly in the FOV. Canon's use of two DOE almost in contact with each other
> > may be an attempt to reduce this problem.
>
> Or maybe one of it's main reasons is to correct the handling of different
> colors. (You know, the first one makes a rainbow out of the colors and the
> next one tries to correct it... same as with classical elements.)
Ok, you were correct. At least that's what Canon wrote on their page; the
other DOE is there to reduce the "superfluous diffracted light." They seem
to correct the chromatic aberrations (couldn't find the first last
time...) with normal refractive element.
> Anyways, the lens was painted white... but that doesn't yet mean it has as
> good optical quality as L lenses. It's not that I would not trust the
> quality of diffractive optics, quite contrary, I don't quite trust this
> classical+diffractive element yet to handle the whole spectrum well...
I still stand behind this... hasn't been proved to be worthy of "L" :-)
Bye,
Hugo.
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** Hugo G�vert **
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