Re: And you thought LF stereo was exotic enough hobby

[rlrylander@xxxxxxx]

Tom Deering wrote:

> >Here's a guy doing lensless stereo using zone plates:
> >http://www.neenas.com/depthography/jones.html
>
> After viewing his page, I still don't understand what zone plates are.  His
> explanation wasn't very clear.  And when I freeview his stereo pairs, they
> look like blurry, poorly matched cha-chas.
>
> I can't say I cared for the images.  Maybe it's like building a model steam
> engine--a historical experiment.
>
> Tom

It is pretty much an experimental excercise rather than a practical way to take
pictures.  Zone plates a highly chromatic "lenses" with poor off-axis
performance, and always form superimposed images with a converging focus at
"+f" and diverging focus at "-f", resulting in poor contrast (some would
interpret this as a desirable "artistic" effect).

Consider a monochromatic point source of light producing concentric spherical
shells of alternating phase.  Cut through this "onion" with a plane at the
desired focal distance "f", and the circles where phase changes from positive
to negative define the edges of the "Fresnel zones".  Block every other zone
(e.g., just the rings where the phase is negative), and you have a "zone
plate".  Illuminate the zone plate with a (same wavelength)  monochromatic
plane wave and you get two sets of spherical waves converging at "f" and
diverging from "-f".  Acts somewhat like a lens, but pretty poor for imaging
applications (mainly used for things like X-ray imaging where you just don't
have proper refractive materials).

Richard Rylander