A medical device for manipulation of material in a tissue includes a sheath, an object engaging unit, and a flexible coupler. The correction coefficient correcting unit corrects a chromatic aberration correction coefficient for a target pixel by using a chromatic aberration correction coefficient for a pixel adjoining the target pixel and accordingly an adequate chromatic aberration correction coefficient is determined. At least one force-transmitting drive element is connected to the bow and to a drive in order to move the sunshade element into a closed setting, with the front end of the sunshade element assuming a predetermined closed position in the closed setting. Moreover, certain embodiments of an ACS may be easily disassembled or collapsed for compact storage. Selecting a segment boundary further comprises selecting one of the candidate boundaries based at least in part on a comparison between two or more of the computed values and determining a boundary within the segmentation window.