  As wireless technology begins to utilize physically larger arrays and/or
higher frequencies, the transmitter and receiver will reside in each other's
radiative near field. This fact gives rise to unusual propagation phenomena
such as spherical wavefronts and beamfocusing, creating the impression that new
spatial dimensions -- called degrees-of-freedom (DoF) -- can be exploited in
the near field. However, this is a fallacy because the theoretically maximum
DoF are already achievable in the far field. This paper sheds light on these
issues by providing a tutorial on spatial frequencies, which are the
fundamental components of wireless channels, and by explaining their role in
characterizing the DoF in the near and far fields. In particular, we
demonstrate how a single propagation path utilizes one spatial frequency in the
far field and an interval of spatial frequencies in the near field. We explain
how the array geometry determines the number of distinguishable spatial
frequency bins and, thereby, the spatial DoF. We also describe how to model
near-field multipath channels and their spatial correlation matrices. Finally,
we discuss the research challenges and future directions in this field.
