Coherent systems are linear in complex amplitude (Amplitude Transfer Function). Incoherent systems are linear in intensity (OTF).

Remember that free space acts as a linear, shift-invariant system. The "Impulse Response" is the Huygens-Fresnel principle.

Use properties like circular symmetry to convert 2D integrals into 1D Hankel Transforms (using Bessel functions). This is often the "shortcut" intended by the author.

Always check your units for spatial frequency (

). In Fourier optics, these are typically in cycles per millimeter.