Upper panel: an astrometric microlensing event illustrated with a close lens (D_L = 50 pc, θ_E = 8.5 mas) and a distant source (according to Fig. 3 of Paczynski 1996a). The lens (red crosses) is moving in front of a background star (big black point) with u₀ = 0.5 (equivalent to 4.25 mas). Due to gravitational lensing there are two images whose paths are shown with blue open circles. If the two images are not resolvable one can measure only the light centroid shift trajectory (violet points) which is an ellipse. Lower panel: different centroid shift trajectories of the same astrometric microlensing event but for different impact parameters u₀ = 0.5, 1, , 2 and 5 (according to Fig. 2 of Dominik & Sahu 2000).