The capillary force between two fine particles or between the tip of an atomic force microscope (AFM) and a surface depends on the precise geometry of the contact region. In this paper we demonstrate that vice versa from a measurement of the adhesion force versus humidity one can calculate the shape of the AFM tip (or the geometry of the contact between particles). This is verified by adhesion experiments with an AFM. (c) 2006 American Institute of Physics.