Kevin Liang and Miguel A. Alonso
in [Opt. Lett. 42(24), 5254-5257 (2017)]
Even for the simplest nontrivial aberration, defocus, and for a circular pupil, there is no theoretical closed-form expression for the optical transfer function (OTF). By using a simple approximation for the error at the pupil, we provide a simple yet accurate approximation for the OTF of a defocused system. We then generalize this approach to generic quadratic aberrations, including astigmatism.