Lens Principal Points

When the medium in front and behind the optical system is the same (in our case it is air), the PPP coincides with the front nodal point (FNP), and the SPP coincides with the rear nodal point (RNP).

For an optical system, the FNP and RNP are two virtual points such that a light ray entering the system and heading straight toward the FNP exits as if emerging straight from the RNP. The angles that the incoming and outging rays form with the optical system’s axis are equal. Figure 1 illustrates.

Figure 1.