  Metal-dielectric layered stacks for imaging with sub-wavelength resolution
are regarded as linear isoplanatic systems - a concept popular in Fourier
Optics and in scalar diffraction theory. In this context, a layered flat lens
is a one-dimensional spatial filter characterised by the point spread function.
However, depending on the model of the source, the definition of the point
spread function for multilayers with sub-wavelength resolution may be
formulated in several ways. Here, a distinction is made between a soft source
and hard electric or magnetic sources. Each of these definitions leads to a
different meaning of perfect imaging. It is shown that some simple
interpretations of the PSF, such as the relation of its width to the resolution
of the imaging system are ambiguous for the multilayers with sub-wavelenth
resolution. These differences must be observed in point spread function
engineering of layered systems with sub-wavelength sized PSF.
