  We report on the observation of strong transmission line shape alterations in
a cold-atom hollow-core fiber interface. We show that this can lead to a
significant overestimation of the assigned resonant optical depth for high atom
densities. By modeling light beam propagation in an inhomogeneous dispersive
medium, we attribute the observations to micro lensing in the atomic ensemble
in combination with the mode selection of the atom-fiber interface. The
approach is confirmed by studies of Rydberg EIT line shapes.
