11c lenses
I don't think there are tubes. At least the lenses in my 11c are a Focotar 60mm and an Elmar 100mm and the "tube" is already built in ie the lenses have long bodies. Come to think of it there are tubes to mount other Leitz and sundry lenses, but don't forget the autofocus facility. The autofocus cam was set up at the factory for the fitted lens; mine came with original lenses, not thought to be amazing by Leitz standards, but at least autofocus is spot on. All you need to do is set the column to thickness of your easel. If easel thickness is exact and autofocus not perfect, put head at top, focus manually, bring head to lowest practical point, re-focus using the fine adjustment screw on the cam; go back to top, re-focus, back to bottom, adjust cam fine adjustment screw. By now should be almost spot-on. If not happy do it once or twice more; now it should be perfect. If your lenses are not factory fitted this autofocus trick should work within reasonable tolerance ie a lens marked focal length 60mm may be 58.75 or 61.334, but you are definitely better off getting original lenses (don't forget 11cs were fitted with several different lenses at factory eg 90mm and 100mm for MF even though pros say these lenses were not up to scratch
Richard