The topography fidelity TFi indicates the accuracy of the estimation of the real surface, describes the instrument influenced deviation of a measured topography image and depends on the interaction of the surface topography with the instrument. To understand and investigate the topography fidelity of optical surface measurement instruments, and interference microscopes in particular, an analytical model based on the fraction of the total illumination criteria and a numerical model based on Richards-Wolf theory are used to characterize the topography fidelity of 3D optical microscopy. As reference artefacts step-like micro-structures with varying spatial frequency and therefore different aspect ratios are numerically investigated with the aforementioned models. To testify the feasibility of the numerical analysis, a commercial white light interference microscope has been employed to measure these reference artefacts. The relationship between the measured heights and the spatial frequency of the samples under investigation are detailed in this paper. The aspect ratio influences on measurement results predicted by the simulation models and the agreements with the experimental results are investigated and reported in detail.