Apparatus and methods are described, for detecting the surface topography of a portion of a curved surface of an object. A beam of light is directed toward the surface from a broad angle of incidence with respect to an optical axis of a camera. Light reflected from the surface is received by the camera, via a narrow-angle aperture. One or more darkened regions in the received light are detected, and the surface topography of portion of the surface is detected at least partially in response to the detected darkened regions. Other applications are also described.