Illustration of the square root of the MinDistance function with a point source behind a Schwarzschild black hole. The black hole and the source are located at the center of the image. The source is at 100M. The darker the pixels, the smaller the distance between the photon and the point source. The color-bar is labeled in M. The minimal and maximal values of the image is about 7 × 10^-4M and 102M (the maximal value of the color-bar is cut at 100M), respectively.