  Astrophysical lensing has typically been studied in two regimes: diffractive
optics and refractive optics. Diffractive optics is characterized by a
perturbative expansion of the Kirchhoff-Fresnel diffraction integral, while
refractive optics is characterized by the stationary phase approximation.
Previously, it has been assumed that the Fresnel scale, $R_F$ , is the relevant
physical scale that separates these two regimes. With the recent introduction
of Picard-Lefschetz theory to the field of lensing, it has become possible to
generalize the refractive description of discrete images to all wave
parameters, and, in particular, exactly evaluate the diffraction integral at
all frequencies. In this work, we assess the regimes of validity of refractive
and diffractive approximations for a simple one-dimensional lens model through
comparison with this exact evaluation. We find that, contrary to previous
assumptions, the true separation scale between these regimes is given by $R_F /
\sqrt{\kappa}$, where $\kappa$ is the convergence of the lens. Thus, when the
lens is strong, refractive optics can hold for arbitrarily small scales. We
also argue that intensity variations in diffractive optics are generically
small, which has implications for the study of strong diffractive scintillation
(DISS).
