  Wireless communication systems have almost exclusively operated in the
far-field of antennas and antenna arrays, which is conventionally characterized
by having propagation distances beyond the Fraunhofer distance. This is natural
since the Fraunhofer distance is normally only a few wavelengths. With the
advent of active arrays and passive reconfigurable intelligent surfaces (RIS)
that are physically large, it is plausible that the transmitter or receiver is
located in between the Fraunhofer distance of the individual array/surface
elements and the Fraunhofer distance of the entire array. An RIS then can be
configured to reflect the incident waveform towards a point in the radiative
near-field of the surface, resulting in a beam with finite depth, or as a
conventional angular beam with infinity focus, which only results in
amplification in the far-field. To understand when these different options are
viable, an accurate characterization of the near-field behaviors is necessary.
In this paper, we revisit the motivation and approximations behind the
Fraunhofer distance and show that it is not the right metric for determining
when near-field focusing is possible. We obtain the distance range where
finite-depth beamforming is possible and the distance where the beamforming
gain tapers off.
