In 1930 the "Leica I Schraubgewinde" was first produced, with an exchangeable lens system based on a 39mm diameter screw thread, often referred to as " Leica Thread Mount" or LTM. In addition to the 50 mm normal lens, a 35 mm wide angle and a 135 mm telephoto lens were initially available. During the mid-1930s, a legendary soft-focus lens, the Thambar 90 mm was designed, and made in small numbers between 1935 and 1949, no more than 3000 units. It is now a rare collector's item.