When looking through a lens of finite aperture, pairs of images associated with caustics, and caustics themselves, are blurred geometrically as well as by diffraction. If the lens aperture is modelled by a Gaussian apodization function, the images in the lens focal plane are Husimi functions of the wave in the plane of the lens, which can be evaluated exactly in terms of Airy and Pearcey functions of complex arguments. Diffraction distorts the spectrum of the source being looked at, even if the geometrical optics is achromatic (as in gravitational lensing); the distortion associated with the images seen as a caustic crosses the lens can be described by the wavelength-dependence of the total light collected by the lens.