 Gravitational-lensing time delay

For a thin lens, relative arrival times follow the difference of $\frac{1}{2}|\theta -\beta {|}^2-\psi (\theta )$ between images, multiplied by $(1+z_l)D_l{D}_s/(c{D}_{ls})$. Both geometry and gravitational potential contribute; an additive delay common to all images is unobservable.