A method of modifying the shape of the modu-lation transfer function of non coherent optical systems based on simultaneous subtraction in real time of the responses of two systems having different pupils is described. When one-dimensional images are treated, the problem is relatively simple, since the second dimension can be used solely for encoding and thus few limitations are posed on the apertures of the optical system. In this paper the design of two-dimensional apertures providing the necessary emphasis of the MTF components in all spatial directions is presented. A circular aperture for one optical system and an annulus one for the second is optimal. Criteria for the determination of the characteristics of these apertures are derived.