Visualization of the “doubling effect”. The observer in this image is directly above the black hole (i.e., the inclination angle is zero). The black-hole’s event horizon is marked by the dark-grey circle, while the unstable-photon region is marked by the light-grey circle. Gravitational lensing enhances the overall size of the lower ring, although the divergence of the rays near the lower ring causes its images to have a reduced thickness. Two images of the lower ring appear; one due to rays that intersect the ring while moving away from the observer, the other due to rays that curve around the black hole and intersect the ring while moving toward the observer. This causes most of the flux that reaches the observer to originate in the lower ring, on the far side of the black hole. Adapted from an image by Alessandro Roussel.