Negative Index Lens Aberrations
  We examine the Seidel aberrations of thin spherical lenses composed of media with refractive index not restricted to be positive. We find that consideration of this expanded parameter space allows reduction or elimination of more aberrations than is possible with only positive index media. In particular we find that spherical lenses possessing real aplanatic focal points are possible only with negative index. We perform ray tracing, using custom code that relies only on Maxwell's equations and conservation of energy, that confirms the results of the aberration calculations. 
D. Schurig and D.R. Smith