> Kate, I don't think I mentioned distortion in my post. A faster ZP has
> more clear rings, but that causes no distortion, the ZP camera still is free
> of linear distortion (at least). The increased number of clear rings
> increase the
> ratio noise/signal, tho. Noise is the light that reaches the film without
> having suffered diffraction, therefore do not contribute to the "sharpness"
> of the image, this is the light mostly responsible for the characteristic
> "glow" of a ZP image. Signal is the light that grazes the edges of the
> rings, suffer diffraction and is "focused" on the film plane, this light is
> the one resposible for the "sharpness" of the image.
> Guillermo
I'm not sure I would describe the workings of a zone plate this way. It
doesn't matter where in the zone plate (or pinhole) the light passes
through. It doesn't bend at one place and not in another. The way you
think about a zone plate or pinhole is that every clear point acts like a
spherical wave is generated there. Then from any point in the image plane,
you add up all the contributions to the intensity at that position from all
the individual contributing points of the zone plate or pinhole. Some
waves will be in phase and reinforce each other, and some waves will be out
of phase and destructively interfere. In the "simple" case of a pinhole,
you discover, that a point source of light in a subject does not generate a
true point of light at the film or even a simple disk, as if a beam of
light came through. Instead, you get the clasic diffraction pattern of a
point, i.e. a central disk (the Airy disk) surrounded by diffraction rings.
Note that every aperture produces diffraction. Even the most perfect lens
can't focus a point source into an image point. Normally you don't see
this because the diffraction pattern is so small. An f/10 lens produces
an Airy disk of only .0134 mm for green light. On the other hand, if you
place a very small pinhole, say 50 microns (.05 mm) 100 mm away from the
film and shine a red laser pointer at it, you will get a diffraction disk
of about 3 mm in diameter.
The zone plate has a lot of chromatic aberration. That is, it can't
focus different colors of light at the same place. That's what causes
the characteristic glow around bright objects in a zone plate photograph.
One color may get focussed sharply and the others will get spread out
into disks of various sizes around the sharp point. I can't speak to the
issue of how much this changes with the number of rings in the zone plate
since I haven't done any research on this. It would be an interesting
and fairly easy experiment to try.
Dick Koolish (kool...@bbn.com)
