Top: astrometric shift for an event with an Einstein radius of θ_E = 12.75 mas (black circle) and an impact parameter of u = 0.75. While the lens (red) passes a background star (black dot, fixed in origin) two images (blue) of the source are created due to gravitational lensing. This leads to a shift of the centre of light, shown in purple for a dark lens. In green, the centre of the combined light is shown for a flux ratio of f_LS = 10. The unlensed centre of the combined light is shown as a red dashed line. The black line connects the current positions of the snapshot. While the lens is moving in the direction of the red arrow, all other images are moving according to their individual arrows. The red, blue, and purple markers correspond to certain time steps (after Proft et al. 2011). Bottom: astrometric shift for different impact parameters. The black dot shows the fixed unlensed source position. The solid lines indicate the shift of the centre of light for a dark lens and the dashed lines indicate the shift of the brighter image. The maximum shift of the centre of light is reached at an angular distance of (purple) (Paczyński 1998), whereas the shift of the brightest image increases continuously with smaller distances.