The Summicron started in 1953 as a 7-element 50mm f/2 lens made in both Leica screw mount and M-mount. A great improvement on its predecessor, the wartime Summitar, the first Summicron was an unusual design with tiny gaps between the normally-cemented front pairs of crown and flint glass lenses, known as "air lenses". Many early Summicrons use thoriated (thorium dioxide) glass which is significantly radioactive and yellows with age. Later lenses use more conventional glass with the same refractive index. This first design of Summicron is notorious for the softness of the front and rear lens elements, which are easily damaged by careless cleaning.
Later on 35mm and 90mm Summicrons followed and the name became standard for all Leica f/2 lenses. The optical design has changed considerably since the first Summicron but has generally been the best available in the range.