Astrometric shift for an event with an Einstein radius of θ_E = 12.75mas (black circle) and an impact parameter of u = 0.75. While the lens (red) passes a background star (black star, fixed in origin) two images (blue dashed, major image + and minor image – ) of the source are created due to gravitational lensing. This leads to a shift in the centre of light, shown in green. The straight long-dashed black line connects the positions of the images for one epoch. While the lens moves in the direction of the red arrow, all other images move according to their individual arrows. The red, blue, and green dots correspond to different epochs with fixed time steps (after Proft et al. 2011).