  Each and every observational information we obtain from the sky regarding the
brightnesses, distances or image distortions resides on the deviation of a null
geodesic bundle. In this talk, we present the symplectic evolution of this
bundle on a reduced phase space. The resulting formalism is analogous to the
one in paraxial Newtonian optics. It allows one to identify any spacetime as an
optical device and distinguish its thin lens, pure magnifier and rotator
components. We will discuss the fact that the distance reciprocity in
relativity results from the symplectic evolution of this null bundle. Other
potential applications like wavization and its importance for both
electromagnetic and gravitational waves will also be summarized.
