Greetings all,   
      
     I'm trying to puzzle out how Nikon and Canon's "diffractive optics" work.    
   These are elements used in some of their telephoto lenses.  The descriptions   
   given on the companies' web sites are not quite specific enough for me.   
     Here's how I think it works.   
     (1) Recall that the term "Fresnel lens" is ambiguous -- it may mean a lens   
   with a spherical surface that has simply been collapsed into several   
   concentric rings, such as a lighthouse lens -- or it may mean a Fresnel zone   
   plate, which is a concentric    
   circular diffraction grating that forms an image.  The reason it forms an   
   image is that the opaque and transparent zones are spaced so as to produce   
   constructive interference at the focal point.  The zones that would cancel   
   them out them are opaque.   
     (2) Canon's and Nikon's diagrams look a bit like a collapsed spherical lens   
   (lighthouse lens) but they are adamant that diffraction is the effect being   
   used.  They show lenses with no opaque zones.   
     So... Maybe they're making something like a Fresnel zone plate, except that   
   instead of being alternately transparent and opaque, the different zones have   
   phase shifts due to different thicknesses of glass.  That would presumably do   
   the job of achieving    
   constructive interference at the focal point.  Just shift half the zones an   
   odd multiple of a half wave, and shift the other half of the zones an even   
   multiple.   
     Is that it?  Does anybody know anything more?   
     There must be a patent in which they say exactly how they're doing it, but   
   it's probably written in Japanese.   
      
   Thanks!   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)