Does the Rodenstock have a lens hood?- just twist the lens hood gently counter-clockwise and unscrew the lens hood. There is a space which will accommodate a Series VI lens filter. Drop it in then screw the lens hood back on. Several manufacturers made threadless auxiliary filters to make it easier to adapt the same filters to different cameras. Series IV,, small, V, bigger, Series VI (fits the Rodenstock) bigger. Make sure to buy good quality clean glass. Zeiss made good lenses. Look for Leica Ernst Leitz Wetzlar Series VI. Kodak Series lenses perform just as well and sometimes you can pick up a full set for very little $$. Other cameras may take a different adapter, but it was a universal size and the filter typically has the size imprinted on the side.