Problems in this section introduce the coherent transfer function (CTF) and the optical transfer function (OTF). A notorious problem: “Compute the OTF for a system with a rectangular aperture and defocus. Plot the result as a function of spatial frequency.” The solution requires integration over overlapping pupil functions—a non-trivial geometric exercise.
(Impulse responses, Fourier transforms, and linear systems).
Remember that a lens introduces a quadratic phase shift: