1. A method of processing an optical image from an optical system, the method comprising: providing the optical system comprising a composite structure comprising an object of interest and an element near or next to the object of interest, the element having a first region with a first transmissivity and a second region with a second transmissivity less than the first transmissivity; irradiating the composite structure with light and measuring a magnitude of a Fourier transform of a complex transmission function of the composite structure; (a) calculating a product of (i) the measured magnitude of the Fourier transform of the complex transmission function of the composite structure and (ii) an estimated phase term of the Fourier transform of the complex transmission function, wherein the complex transmission function equals or approximates a minimum-phase function or a maximum-phase function; (b) calculating an inverse Fourier transform of the product, wherein the inverse Fourier transform is a spatial function; and (c) calculating an estimated complex transmission function by applying at least one constraint indicative of a known property of the complex transmission function to the inverse Fourier transform by making the inverse Fourier transform conform to the known property.