We have considered thus far the diffraction of fields according to Huygen’s principle. We decompose the field at each plane into a superposition of spherical waves, and then we can propagate the fields by allowing those spherical waves to expand. The next step is to consider what happens when we introduce a lens into the optical path. Our treatment of the lens will follow the treatment of Goodman. At every point in the lens we consider the thickness of the lens, and treat the lens as something that introduces a phase delay in the fields that pass through that point.