The Tessar is a famous photographic lens design conceived by the German physicist 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.

A Tessar comprises four elements in three groups, one positive crown glass element at 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.

Despite common belief, the Tessar was not developed from the 1893 Cooke triplet design by replacing the rear element with a cemented achromatic doublet. Paul Rudolph designed the Anastigmat with two lenses cemented in 1890. Later, Rudolph thought that a narrow airgap in the form of a positive lens would correct the spherical aberration (as did HL Aldis in 1895) and that this device was much better than the lenses cemented. In addition, this allowed the photographers to have greater freedom when choosing the lenses. In 1899 he separated the lenses in the Anastigmat to produce the fourth element, a group of four Unar lenses (which replaced the two interfaces cemented by the aforementioned device). In 1902, he realized that 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 the 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; reduce the gap found between astigmatic foci.

The first Tessar appeared with a maximum aperture of f/6.3, but by 1907, the aperture had been reduced to f/4.5. In 1930, Ernst Wanderslab and Willy Merté from Zeiss developed Tessar lenses with apertures of f/3.5 and f/2.8.

Tessar lenses have been created by millions, by Zeiss and other manufacturers, and are still produced as excellent intermediate aperture lenses. The famous 50mm f/3.5 Elmar lenses used in the first Leica cameras were of this type, designed by Max Berek in 1920. Actually, Zeiss has a large monopoly on this type of construction, because Rudolph's patent was very general. His only claim was:

"A spherically, chromatically and astigmatically corrected objective consisting of 4 lenses separated by the diaphragm into two groups, each of two lenses, of which group 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".

This page was last edited on 31 March 2018, at 23:42.
Reference: https://en.wikipedia.org/wiki/Zeiss_Tessar under CC BY-SA license.

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