This paper presents a new method of image edge enhancement. A unit-amplitude, normal incident plane wave illuminates two superimposed transparencies, which situated in front of a converging lens. One transparency is correct image, and another is out-of-focus image. In the back focus plane of the lens, the interference fringe of Fourier spectra of two transparencies formed by the lens are filtered by slit. In the output plane of the optical processing system, an edge enhanced image is obtained.