I've just bought a sparkling 50mm 1.4 MC RokkorX with a full set of extension tubes for macro. With the 50 the lens w/all 3 (12-20-36mm) is nearly unfocusable ( <1" away). It's do-able with the 12 0r 20mm seperately but the magnification is not great. Would a different focal length be more manageable as far as distance to object? How would a 28mm work? 100mm? BTW, what are the spring loaded sleeves on the tubes for?