This paper deals theoretically with the influences of various kinds of mutual co-herence function and the degree of coherence on the distribution of straight-edge generated image field. Indicating the close connections between the illuminating con-dition and image formation, we thus obtain the reliable method to select the optimal illuminating condition. Based on our theoretical calculation on optical system with aberration, it is shown that the radial intensity distribution of the straight-edge generated imagery can reflect sensitively the quality of the optical system. This can be adopted as the quality criterion for practical optical system under partially cohe-rent illumination.