> --- "Gordon J. Holtslander" <hol...@duke.usask.ca> wrote: > > I'm going to try making some zoneplates. I've read about that zoneplates > > require focussing. None of my cameras pinhole cameras can focus. Is > > zoneplate focusing necessary in all cases? Is focusing needed only for > > subjects that are close to the camera? > > > > Does it matter whether a positive or negative image of the zone plate is > > used? Does the center circle have to be opauque?
Here is what I think: Unlike pinholes which are "a-focal", zoneplates do focus light by virtue of diffraction, therefore, technically speaking you 'd need to focus them. In practice, it is so difficult to do it and according to my tests, unnecessary. Just make the ZP for the "infinity" focusing distance, mount it at a distance equal to the focal length used to calculate it and that should work for shooting distances of 2 or 3 feet to infinity. About your second question: ZP work by suppressing destructive diffraction by consecutive rings. As long as we blacken every other ring, the constructive diffraction should happen, no matter the order (clear or dark center ring). Having said that, IMO, a better image can be obtained (haven't really experimented) by having a clear center ring. After all, the clear center ring is nothing but a "pinhole", so we have a pinhole image (good "geometric" image) and a superposed diffraction image that makes the lens faster and have the characteristic "glow" of ZPs. Guillermo
