The problem of a perfect lens made of a slab with negative refraction
  The problem of the principal existence of the perfect lens and superlensing is discussed. We have demonstrated that in the case of the virtual focus the idea of perfect lens based upon amplification of evanescent waves as proposed by Pendry is perfectly right unlike the case of the real foci. We think that some experimental results claiming superlensing can be explained in terms of the proposed theory for the case when the virtual focus is inside the lens but very close to the rear face of it. 
A. L. Efros