> Wouldn't a significant part of the weight savings come from a reduction in
> the amount of glass in the lens? It seems to me that a DO lens could be
> extremely thin, since the light-bending is being done by the diffraction
> grating rather than by the refraction of a varying thickness of glass. In
> other words, the DO element could be essentially flat, as opposed to a
> conventional element that's convex, concave, or some combination thereof.
>
The glass is typically a small part of the overall weight of lenses. The
main weight savings comes from the extra degrees of freedom that allow the
optical designer to reduce the length of the lens. Look at the 200 mm
F/1.8, 300 mm F/2.8 and the 400 mm F/4 (the unreleased lens the DOE lens was
compared to). Each lens is approximately 100 mm in diameter and similar
design, but as the focal length increases the length and weight increases.
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