A while ago I saw a product called "digilens"
that was 3 ? diffractive lenses in one lens holder.
You could switch between them with an address line.
The optical properties of this optic were programmed at the factory.
The one I saw (photo) had 2 positive and one negitive lenses of
different focal lengths IIRC

Dan


wrote:
"Is it possible to construct a telescope using
holographic lens such that you flick a switch and the lens
just formed at the front."

Not quite, but about 15 years ago I read an article in Advanced Optics
that whowed if you took a 1 wave reflector, a laser, and a holographic
plate, that you could print the wavefront error on that plate. Then
later after the plate was developed, you could use the plate to convert
the scope from a 1 wave monster into a 1/3 wave--almost
tollerable--scope. However, the colimation tollerances with respectof
plate to optical axis were "really stiff".

Mitch