In this paper we report on the effect that a single fluorine substitution in a hydrocarbon chain of stearic acid has on the molecular ordering of the material. and, in addition, the effect that the fluorines have on the scanning tunneling microscopy (STM) contrast of an alkyl chain physisorbed on graphite. The area demand due to a single fluorine substitution is increased by only approximately 1%, demonstrating a very similar steric impact of hydrogen and fluorine. The required accuracy in the measurement is achieved by evaluating the moire pattern due to the adsorbate and substrate lattices. The predicted change in the STM contrast caused by the fluorine atom was observed for the first time.