Thanks, all.  I have just looked at Miyamoto's paper in JOSA 1961, and it   
   sounds like I guessed right.  At least, I guessed one way to make it work.   
      
   There is one bothersome anomaly.  What Miyamoto *describes* is what I guessed   
   -- a Fresnel zone plate in which the odd-numbered zones are not black, but   
   rather thickened to add half a wave of phase shift.  (And he varies the   
   parameters so that it    
   simulates aspheric rather than spherical elements.)  His *diagrams*, however,   
   look like refractive Fresnel lenses, complete with slanted surfaces in the   
   zones.  That is, his pictures look like an aspheric refractive lens being   
   simply flattened like a    
   lighthouse lens.   
      
   That may be the source of the lighthouse-lens-like diagrams that still appear   
   in manufacturers' web pages about diffractive optics.   
      
   Miyamoto worked for both Nikon and Kodak at the time.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)