The efficiency of filter media can be improved by applying nanofibers to the surface of the microstructure. In order to gain a deeper insight into the influence of the nanofiber geometry on filtration efficiency in terms of pressure loss, separation efficiency, and dust holding capacity, the microstructure is analyzed by geometrical pore space characteristics such as the degree of vertices function, the spherical contact distribution function, and the pore size distribution function. The pore space characteristics are investigated by using a distance transform and a 3D pore space graph generated by the Ma-Sonka-Algorithm with the P-simple-points extension.