Figure 4.11 — Focusing of an optical beam Figure 4.11 — Focusing of an optical beam. The beam crosses an aperture stop, or iris, with radius R and then propagates towards the principal plane p1 and is transferred to the principal plane p2. At p2, the beam is refracted and focused towards the focal point O. The point R is the intersection point of a ray with p2 and illustrates the relation between the position ρ of the incident ray at p1 and the propagation angle θ of the transmitted ray at p2 , i.e., Abbe’s sine condition [Eq. (4.39)].