  Perfect lensing using negative refractive index materials and radiationless
electromagnetic interference both provide extreme subwavelength focusing by
"amplifying" evanescent wave components that are usually lost. This paper
provides a relation between the achievable focus spot size, the amplification
available and the focal length. This may be considered as a revised version of
Abbe's diffraction limit for focusing systems that have evanescent wave
amplification. It is useful in comparing the amplification achieved in various
subwavelength focusing implementations, as well as determining when it is
better to use existing near-field techniques, such as simple diffraction from
an aperture or slit, than to attempt complicated superfocusing.
