Zeiss Tessar
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The Zeiss Tessar is a famous photographic lens design conceived by Paul Rudolph in 1902.
The Tessar was not evolved from the 1893 Cooke triplet design, as commonly believed, by replacing the rear element with a cemented achromatic doublet . In fact, in 1890 Paul Rudolph designed the Zeiss Anastigmat with two cemented doublets. Later, in 1899, he took apart the doublets in the Zeiss Anastigmat to produce the four element, four group Unar lens. In 1902 he realized that to reverse the two rear elements of the Unar and go back to a cemented doublet would improve the performance of Zeiss Unar lens. He named the result "Tessar", from the Greek word τέσσερα to indicate a four element design.
A Tessar comprises four elements in three groups, one positive crown glass element on the front, one negative flint glass element at the centre and a negative flint glass element cemented with a positive crown glass element at the rear.
Early Tessar designs allowed a maximum aperture of f/6.3. Later development allowed an aperture of f/4.5 by 1917, while further improvements with lanthanium glass allowed an aperture of f/3.5. Later evolution of the Tessar gave a maximum aperture of f/2.8, but the picture quality at full aperture was significantly inferior to a f/3.5 Tessar.
The Tessar design patent was held by Zeiss for two decades, though the design was licensed to Bausch & Lomb in the United States and to Krauss in France.
Front element focusing Tessar
The front element of the Tessar has three times the power of the whole lens, requiring only one third the amount of displacement to achieve focusing, and that the Tessar lens has a large airspace between the first and second elements, require that only front element movement is required for focusing. Since the displacement is small compared with the airspace, the adverse effect on image performance is not severe. The front element focusing Tessar has the advantage of lower cost due to simplicity and thus was widely used in many midrange Zeiss Ikon cameras.
A front element focusing Tessar does not permit the use of a polarization filter, because of front element rotation during focusing.
The unit focusing Tessar, although much more complicated in mechanical construction, offers still better performance. It was used on higher end cameras such as Contaflex Super B, Rolleiflex TLR, etc.
Unit focusing Tessars were also used on a few Medium Format folding cameras, like the Agfa Super Isolette, and its Russian clone, the Iskra.
Pro Tessar
The front element of the Tessar can be replaced to make a telephoto or wide angle lenses. In 1957 Carl Zeiss offered the Pro Tessar 85mm f/4 and Pro Tessar 35mm f/4 for use on their Zeiss Ikon Contaflex cameras.
Reference
A History of the Photographic Lens, Kingslake, Rudolf ISBN 0-12-408640-3de:Tessar pl:Tessar zh:天塞镜头 ru:Тессар