Fig. 11.

Definition of optical invariant. (a) Optical system with effective focal length $f_0$ modeled with first-order principal planes. The refractive indices in image and object space are labeled as $n$ and $n^{\prime }$, respectively. Front and rear focal lengths depend on the refractive indices. Back focal plane and aperture stop coincide. Marginal and chief rays are shown to visualize FOV, NA, and optical invariant. Also shown is the model of the same system with a thin lens. (b) Same as (a), but for aplanatic model. Refracting surfaces are spherical. This definition is also known as the Abbe sine condition.