1. A method of processing an optical image, the method comprising: (a) using an optical image processing system to calculate a product of (i) a measured magnitude of a Fourier transform of a complex transmission function of an object or optical image 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 of 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.