Absolute value of the magnification for the two images of a source at redshift z_s = 2 as a function of redshift of SIS2. The lens SIS1 is kept fixed at redshift z_SIS1 = 0.5 and SIS2 is free to move along the optical axis within redshift z_SIS2 = 0.8 and z_SIS2 = 1.5. The predicted magnification shown here corresponds to the multi-plane model, but we observe the same trend for the model with a single lens.