Variation of the magnification above baseline A − 1 and the normalised centroid shift δ_mic/θ_E during a microlensing event as a function of the normalised source-lens separation u. Continuous and dotted black curves correspond to the cases of a dark lens (f_L = 0) and a luminous lens with f_L/ f_S = 1, respectively. The asymptotic behaviour of the photometric and astrometric signals for the specific case of a dark lens is plotted using red and blue curves, respectively, and these asymptotic curves are labelled with their functional form.