Schematic representation of a gravitational lens system. The observer is at the left, the source at the right. The lens is composed of multiple lensing planes, L_i. A light ray (red) originating from the source plane S is deflected by the lensing planes and the associated image is formed at the observer’s position along the direction θ. The blue line in the direction β is the position that the source would have in the absence of lensing. The deflection angles ${\widehat{\alpha }}_i$ are computed at the angular positions x_i of the images. All distances needed for the computation of the relation between θ and β are marked at the top of the figure.