We discuss some of the intrinsic difficulties related to the numerical investigation of near-held optical microscopy. We show how these difficult ies can be handled within the framework of the Green's tensor technique. Investigating practical experimental configurations, we show the strong correlation between the motion of the tip and the near-field signal. In order to investigate this kind of artifacts, we propose a series of experiments based on photonic band gap systems immersed in evanescent fields. The electromagnetic properties of such a system can be changed without changing the the topography of the system at all.