I'm working on some macro and micro experiments and a question came up that I've never really had to think about before and it has me stumped. I'm using a 120mm Rodenstock Macro lens and in my experiments I've played with putting some Hoya close up lenses (+1, +2, +4) on for additional magnification. My question is does the use of a close up lens change the actual focal length of the lens being used? This came up because when I go to compensate for bellows length I need to know the focal length of the lens to figure out my compensation. Does the close up lens change my focal length or is it factored in some other way? Thank you.