Practical resolution of optical lithography is often defined as the minimum feature size which can be fabricated with acceptable depth of focus. It can be predicted by calculating diffraction image contrast in various defocused planes. However, since a large volume of calculation is required to know image contrast under partially coherent illumination, evaluation of optical lithography systems with regard to use of the practical resolution is time consuming and does not give us a quick and clear forecast on optimum optical parameters for a given lithography specification. In this paper, we propose an analytical and intuitive method for getting image contrast in defocused planes, by use of the theory of interference fringe formation. By using this method, relations among defocus, numerical aperture, wavelength, coherence factor, and image contrast are derived analytically and these parameters can be optimized for given lithography methods, which employ not only the conventional but also various resolution- enhanced methods, such as annular illumination, phase-shift, illumination control, and others.