it's the radius of the event horizon. Where the escape velocity is the speed of light. A photon will not orbit a black hole at the event horizon. it would skirt the hole and speed away.

some equations:

basic force equation.

The approximate equation for the force of gravity between 2 objects.

The equation for centripetal force.

the equation for the Schwartzchild radius.

Where F is the force, a is acceleration,

is the mass of the object you are attracted to,

is your mass, and

is the schwartzchild radius of the object you are attracted to.

Given these useful tools, we can calculate the radius at which an object will endlessly orbit an object by playing with the first three equations:

and

so

notice that they tie into the fourth, like so