In this ≪ contribution we address the question to what extent surface
charges affect contact-mode scanning force microscopy measurements. % We
therefore designed samples where we could generate localized electric field
distributions near the surface as and when required. % We performed a series of
experiments where we varied the load of the tip, the stiffness of the
cantilever and the hardness of the sample surface. % It turned out that only
for soft cantilevers could an electrostatic interaction between tip and surface
charges be detected, irrespective of the surface properties, i.\,e. basically
regardless its hardness. % We explain these results through a model based on
the alteration of the tip-sample potential by the additional electric field
between charged tip and surface charges