Objective quantification of lens images is essential for cataract assessment and treatment. In this paper, bottom-up and top-down strategies are combined to detect the lens contour from the slit-lamp images. The center of the lens is localized by horizontal and vertical intensity profile clustering and the lens contour is estimated by fitting an ellipse. A modified active shape model (ASM) is further applied to detect the contour of the lens. The average intensity inside the lens is employed as the indicator of nuclear opacity. The relationship between our automated nuclear cataract assessment and the clinical grading is analyzed. The preliminary study of forty images shows that the difference between automatic grading and clinical grading is acceptable.