  We show that a single-interface between regular and double-negative (DNG)
media constitutes the core structure of a perfect lens and, furthermore,
substantially outperforms the highly discussed DNG slab-based lens under
imperfect conditions, maintaining unbounded resolution. We identify the
perfect-lensing mechanism as the excitation of a Brewster (real improper) mode
at the interface - a mode with a completely flat-band dispersion at
media-matching conditions, perfectly transferring all incident waves including
evanescent. A lossless DNG slab retains the single-interface perfect-lensing
characteristics solely because such a Brewster mode excited at the front
(input) slab interface merges in a non-interacting fashion with a confined
surface-mode at the rear (output) - thereby preserving its original flat-band
dispersion. While the single interface perfect lens retains its high resolution
under loss, imperfect media-matching and off-resonance excitations, the DNG
slab lens performance rapidly deteriorates under such conditions due to
excitation of the symmetric and anti-symmetric slab surface-modes.
