What spherical zone around a black hole can contain circular photon orbits?

The spherical zone around a black hole that can contain circular photon orbits is the photon sphere.

In a non-rotating Schwarzschild black hole, the photon sphere lies at 1.5 times the Schwarzschild radius, or 3GM/c² from the center. At this distance, light can follow a circular path because spacetime curvature bends its trajectory strongly. The orbit is unstable: a tiny disturbance sends the photon either outward or into the black hole.

The event horizon and photon sphere are not the same thing. The event horizon marks the boundary beyond which light cannot escape, while the photon sphere is farther out. A rotating Kerr black hole does not have one perfectly spherical photon zone; frame dragging produces direction-dependent photon orbits. The bright ring seen in black-hole images is related to strongly lensed light near these photon trajectories, but it is not simply a photograph of the photon sphere itself.