Tessar
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The Tessar is a famous photographic lens design conceived by physicist Paul Rudolph in 1902 while he worked at the Zeiss optical company. Since Zeiss was the patent holder of the design, this type of lens is commonly known as a Zeiss Tessar.
Despite common belief, the Tessar did not evolve from the 1893 Cooke triplet design by replacing the rear element with a cemented achromatic doublet. In fact, in 1890, Paul Rudolph designed the Anastigmat with two cemented doublets. Later, in 1899, he separated the doublets in the Anastigmat to produce the four-element, four-group Unar lens. In 1902 he realized that reversing the two rear elements of the Unar and returning to a cemented doublet would improve the performance of the 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 center and a negative plano-concave flint glass element cemented with a positive convex 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 lanthanum 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 for those lenses is significantly inferior to that of 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.
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[edit] 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 air space 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.
The unit focusing Tessar (where the whole lens assembly moves in relation to the film for focusing), although much more complex in mechanical construction, offers still better performance. It was used on higher-end cameras such as Contaflex Super B, Rolleiflex TLR, and larger format plate cameras like the Maximar.
[edit] 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 the central shutter SLR Zeiss Ikon Contaflex cameras.
[edit] Vario Tessar
The Vario Tessar name is used by Zeiss for various zoom lenses including the digital still camera Sony DSC-P100 released in 2004.
[edit] References
A History of the Photographic Lens, Kingslake, Rudolf ISBN 0-12-408640-3
