  Negative index of refraction has become an accepted part of transformation
optics, which is encountered in transformations that change the orientation of
the manifold. Based on this concept, various designs of perfect lenses have
been proposed, which all rely on a folding of space or spacetime, where the
maps from electromagnetic space to laboratory space are multi-valued. Recently,
a new concept for perfect imaging has been proposed by Leonhardt and Philbin,
which also uses multi-valued maps, but does neither include negative index of
refraction nor an amplification of evanescent modes. In this context it was
speculated that multi-valued maps should be seen as the basis of perfect
imaging rather than amplification of evanescent modes. It might be useful to
review the standard lens based on negative index of refraction from this point
of view. In this paper we show that a negative index of refraction is not an
inherent characteristic of transformation optics, but rather appears as a
specific choice of a sign ambiguity. Furthermore, we point out that the
transformation designed lens does not amplify evanescent modes, in contrast to
the Pendry-Veselago lens. Instead, evanescent modes at the image point are
produced by a duplicated source and thus no imaging of the near field takes
place.
