LR broad-band properties are sensitive to electronic transport and charge localization. This allows to investigate the electronic structure of ultrathin metal films by means of IR-transmission spectroscopy. As this electronic structure is determined by the interfaces of the film, the dynamic conductivity also reflects its morphology. We performed in-situ transmission spectroscopy in the middle infrared (MIR) during the evaporation of Fe on MgO(001) at room temperature in ultrahigh vacuum. We varied the quality of the substrate surface by using MgO(001) cleaved in air and cleaved in ultrahigh vacuum. The difference in the thicknesses for the onset of continuous transport properties of the growing films is well detected by their different IR-spectroscopical behavior. For continuous films, we calculate the thickness dependent scattering of electrons and the thickness dependent effective IR-optical oscillator strength from their IR-optical data. For MgO cleaved in air compared with cleavage in ultrahigh vacuum, the continuous thin film model becomes relevant at a smaller thickness, i.e. the substrate is covered faster in this case.