> 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).
>
Normally, the lens designer has only a limited number of _things_ that he
can control to reduce aberrations. This includes the types of glass, the
curvature of each surface, the thickness of elements and their spacing.
Sometimes adding aspheric surfaces helps. When none of this helps, the
designer must resort to adding more (lens) elements or exotic glasses. Or
reduce the lens specs.
The DOE gives the optical designer new degrees of freedom to help correct
aberrations in the optical system. Specifically, the element can be
designed to be behave as an asphere and the dispersion of a DOE is radically
different from any possible glass lens. An early applications of
diffractive optics combined both effects to change a simple, plano-convex
lens into a diffraction limited achromatic lens.
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