The Tessar is a photographic lens design conceived by the German physicist Dr. Paul Rudolph in 1902 while he worked at the Zeiss optical company and patented by Zeiss in Germany; the lens type is usually known as the Zeiss Tessar. Since its introduction, millions of Tessar and Tessar-derived lenses have been manufactured by Zeiss and other manufacturers, and are still produced as excellent intermediate aperture lenses. The Tessar design uses four spherical lens elements in three groups, one positive crown glass element at the front, one negative flint glass element at the center and a negative concave flint glass element cemented with a positive convex crown glass element at the rear.
History
Beginnings Despite common belief, the Tessar was not developed from the 1893 Cooke triplet design, although it appears the Tessar replaces the single rear element of the Cooke triplet with a cemented achromatic doublet. Instead, the Tessar underwent a parallel evolution from Paul Rudolph's 1890 Anastigmat lens, which had four elements in two cemented groups. Hugh L. Aldis patented the Stigmatic lens line for Dallmeyer in 1895; in one implementation, the front group from the Anastigmat design was modified by adding a narrow air gap, which acted as a positive element and improved zonal correction. Later, Rudolph adopted the same device to modify the Anastigmat design, resulting in the Unar of 1899. In addition, this allowed the photographers to have greater freedom when choosing the lenses. In one implementation, the Unar has four air-spaced elements in four groups, which replaced the two cemented interfaces of the earlier Anastigmat design. In 1902, Rudolph realized the two cemented interfaces had many virtues, so he reinserted them in the back of his Anastigmat, maintaining the "air gap" of the previous part of the Unar, thus creating the Tessar design (from the Greek word τέσσερα (téssera, four) to indicate a design of four elements) of 1902. The frontal element of the Tessar, like that of the Anastigmat, had little power since its only function was to correct the few aberrations produced by the powerful posterior element. The set of interfaces cemented in the posterior element had 3 functions: to reduce the spherical aberration; reduce the overcorrected spherical-oblique aberration; and reduce the gap found between astigmatic foci.
Improvements and evolutions The first Tessar appeared with a maximum aperture of f/6.3, but by 1917, the maximum aperture had been increased to f/4.5. In 1930, Ernst Wandersleb and Willy Merté from Zeiss developed Tessar lenses with apertures of f/3.5 and f/2.8. In 1925, E. Wandersleb and W. Merté of Zeiss created the Biotessar consisting of two elements cemented in the front, a single negative element in the center, and three cemented in the rear. After World War II and the partitioning of Germany, the Zeiss factory at Eisfeld ended up in East Germany; Zeiss Jena developed a popular camera line named the 'Werra', after the Werra river which runs through the town. Many models were equipped with Tessar lenses, which were marked as "Zeiss-Tessar", resulting in legal action from the Zeiss company in Western Germany. For a while the Werra Tessar lenses were marked simply as "T", but eventually they were allowed to market the lenses as "Carl Zeiss Jena Tessar".
Tessar-derived lenses
Zeiss had strong control over the Tessar design, because Rudolph's patent was very general. In the corresponding U.S. Patent, he claimed:"A spherically, chromatically and astigmatically corrected objective, consisting of four lenses separated by the diaphragm into two groups each of two lenses, of which groups one includes a pair of facing surfaces and the other a cemented surface, the power of the pair of facing surfaces being negative and that of the cemented surface positive." —Paul Rudolph, US Pat. 721,240 The Tessar design patent was held by Zeiss for two decades, and licensed to Ross in the United Kingdom, Bausch & Lomb in the United States, and to Krauss in France. Only licensed manufacturers were allowed to use the brand name Tessar. Many other manufacturers tried to copy the design of the Tessar lenses but due to the breadth of the patent, they could not. The simplest way was to use a cemented triplet for the rear group instead of a doublet. In 1913, many designs of this type appeared, including the Ross Xpress by J. Stuart and J.W. Hasselkus, Gundlach Radar, and Berthiot Olor by Florian. After the patent expired, Tessar-derived lenses were widely made by many manufacturers under different trade names. For example, the Minoxar 35/2.8 lens on the Minox M.D.C and GT-E is the fastest and widest Tessar-type lens achieved so far by using lanthanum glass elements. The picture quality was outstanding. Other Tessar-type lenses include:
Leitz Elmar
It is sometimes believed the Leitz Elmar 50 mm f/3.5, designed by Max Berek in 1920, was derived from the Tessar, as they share the same general layout. The Elmar lenses were used in the first Leica cameras. Although the Tessar and Elmar lenses appear similar in layout, there is a lot more to the design and performance of a lens than simply the layout of the glass elements. The position of the stop, the optical characteristics of the glasses used for each element, the curvature of each lens surface, and the negative format that the lens is designed to cover, are all vital to the performance of the lens, and in the Leica lens these were all different from the Tessar. When the Leica was being developed, Oskar Barnack tried a 50 mm Tessar, but because it had been designed to cover only the 18×24 mm field of a cine frame, he found it inadequate for coverage of the Leica 24×36 mm format. The lens designed by Max Berek for the Leica rangefinder camera was a modified Cooke triplet with five elements in three groups, the third group being three cemented elements, with the aperture stop in the first air space. This lens, called the Elmax, gave good coverage of the 24×36 mm format and was used until improved optical glass allowed the third group to be simplified to a cemented pair when it was renamed Elmar. It was not until Zeiss Ikon was developing the Contax camera to compete with the Leica that the Tessar was redesigned to cover a 24×36 mm negative.
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