The Sonnar is the best and fastest lens used on the Rollei 35's. It was designed by Carl Zeiss with five elements in four groups and has Rollei's HFT multi-coating. The Sonnar is one of the finest lenses of the 20th Century designed in 1930 by Ludwig Bertele, one of the finest lens designers of the 20th Century. Bertele originally worked for Ernemann, one of the premier German optics houses. When Ernemann was absorbed into the Zeiss-Ikon combine in 1926, Bertele began working for Carl Zeiss. The original was an uncoated f/2 5cm focal length with six elements in three groups specifically created for the Zeiss Ikon Contax. In 1932 it was reformulated for an f/1.5 5cm with seven elements in three groups. At some point after the Zeiss Ikon Contax IIa/IIIa ceased production in 1961, the Sonnar was reformulated into the configuration commonly found now using five elements in four groups for a slower f/2.8 lens. However, its design principles are essentially the same as the original Sonnar. The Carl Zeiss Sonnar has been used as the premier lens on numerous 35mm cameras. As with the Tessar, from the 1960's and beyond it is normally found in the slower f/2.8 five element, four group configuration.
Rollei wanted a faster lens for its top Rollei 35 model and Carl Zeiss reformulated the Sonnar using Rollei's HFT multi-coating resulting in the 40mm f/2.8 with five elements in four groups. It is a stunning lens noted for near zero distortion in its very flat field, very low falloff, exceptional resolution and very high contrast; all the attributes sought for in a superb lens. As with the later Tessar's the Sonnar HFT was manufactured by Rollei under license from Carl Zeiss. Compared to the Tessar, the Sonnar is a better, faster lens but is much more difficult and expensive to manufacture. The Sonnar has extremely tight tolerance requirements for its complex element shapes and their spacing. In spite of the age of its design, it is still a world class lens and holds its own quite easily with the very best of modern lenses.
