I have a 300mm Fuji lens that I would like to use on my old speed graphic at som ething other than infinity. I've seen prevous posts reccomending using a nikon T series closeup lens (5T/6t) with a Nikon 300mm. Does adding a diopter somehow "decrease" the focal length? HOw does that work? The nikon catalog is little he lp. I need a 52mm size diopter and does B&W or Rodenstock or Canon or anyone els e may a good quality one in that size or is nikon the only one? Your help would be appreciated.