The Rodenstock 138mm/6.5 is unusual for a larger-format lens in that it has a floating internal element that has to move as it’s focused. This means it has to be focused with a helical ring (like an SLR lens) that moves both the focus and the internal element, rather than simply by moving it back and forth with a bellows. This makes the lens big, heavy, and cumbersome (not to mention spendy), but it’s an extremely sharp design.