  An analysis of null geodesics in Schwarzschild de Sitter space is presented
with special attention to their global `bending angles', local measurable
angles, and the involvement of the cosmological constant. We make use of a
general technique which allows for finding observable intersection angles of
null trajectories analytically. A general relativistic aberration relationship
is established as one of its applications. The question of whether or not the
cosmological constant, $\Lambda$, contributes to orbits of light and to related
observable quantities is addressed in detail. We also discuss the ongoing
debate on this issue and respond to some recent papers on the topic. The
dependence of measurable quantities on the motion of observers is stressed
throughout. Exact formulas for measurable intersection angles, as well as
gravitational lens equations for observers in the Schwarzschild de Sitter
background are provided.
