In this work, measurement in the near-field region of a sub-wavelength separation between
two atoms is studied when the detector used to perform the measurement is modeled as
another atom. Through an analysis of multiple scattering of an incident wave among the
atoms, the back-action of the detector on the measurement is examined. It is found that,
although the separation can never be measured exactly, the presence of the back-action
makes the measurement more accurate. It is also found that the back-action enables the
waves observed by the detector to avoid singularities that otherwise exist when the
back-action is ignored.