The optical appearance of a black hole universally features a thin, bright "photon ring" which is composed of an infinite sequence of self-similar, increasingly demagnified images of the accretion disk. These images result from light rays that orbit the black hole numerous times before reaching the telescope. While the emission profile strongly depends on the disk's details, the lensing pattern is universal and is determined by General Relativity (GR) alone. In the talk I will review the structure of the photon ring of rotating (Kerr) black holes. I will then describe a suggestion for measurement of the demagnification pattern of successive subrings using future extensions of the Event Horizon Telescope. This will enable precision spin inference and provide a new, independent strong-field test of GR. I will then describe some surprising potential optical signatures of rapidly rotating black holes and comment on future prospects.
