
4
STOPS, PUPILS, AND WINDOWS
Thus far, as we have considered light propagating through an optical system, we have assumed
all surfaces to be innite in the dimensions transverse to the axis. Thus there has never been a
question of whether a ray would or would not go through a particular surface. This assumption
proved useful in graphical solutions where we worked with three rays in Figure 2.1. As we used
the ray parallel to the axis, for example, we did not trouble ourselves by asking whether or not
this ray passed through the lens, or was beyond its outer radius. We knew that all the rays
from the object, regardless of their angle, would go through ...