A 50mm lens which has its own focusing helicoid and which goes from 1:1 or 1:2 to infinity is more correctly referred to as a purpose built macro lens. A true macro lens is one which has no helicoid, which is optimized for a limited range of magnification and which has an RMS thread mount. The Zeiss Luminars and Leitz Photars and Macro Summars are the best known of this type. In some cases the shorter lenses were in RMS mount while the longer ones were in Leica 39mm mount. This is true of the Macro Nikkors, not to be confused with the oddly named Micro Nikkors.