Page "Gravitational redshift" Paragraph 14

the distance between the center of mass of the gravitating body and the point at which the photon is emitted.

The redshift is not defined for photons emitted inside the Scharzschild radius, the distance from the body where the escape velocity is greater than the speed of light.

Therefore this formula only applies when is at least as large as.

When the photon is emitted at a distance equal to the Schwarzschild radius, the redshift will be infinitely large.

When the photon is emitted at an infinitely large distance, there is no redshift.