  We discuss and measure the phase shift imposed onto a radially polarized
light beam when focusing it onto an $^{174}\text{Yb}^{+}$ ion. In the
derivation of the expected phase shifts we include the properties of the
involved atomic levels. Furthermore, we emphasize the importance of the
scattering cross section and its relation to the efficiency for coupling the
focused light to an atom. The phase shifts found in the experiment are
compatible with the expected ones when accounting for known deficiencies of the
focusing optics and the motion of the trapped ion at the Doppler limit of laser
cooling.
