Protar (lens)
Protar is the name of a type of four-lens, two-part camera lens developed by Paul Rudolph for Zeiss in 1890 . It was the first anastigmat . Protare were considered to be very good wide angle lenses at the beginning of the 20th century.
History of construction
The development of barium- containing crown glass by Schott in 1886 made it possible for the first time to construct anastigmatic lenses without field curvature, because this glass (e.g. Schott BaK4) has a significantly higher refractive index than "normal" crown glass and also than some flint glasses , so that you can build an achromatic lens from barium crown glass and flint glass .
On the basis of the barium crown glasses and a telescope design calculated by Carl Friedrich Gauß in 1840, Paul Rudolph constructed in 1889/1890 the first of a series of four-lens, two-part lenses that were based on an astigmatic correction principle for image field flattening. The result was the first lens worldwide that showed neither field curvature nor astigmatism . A German imperial patent with the number 56109 was granted on April 3, 1890.
Around 100,000 copies of these anastigmats were sold in the first 10 years. Two other lens types were derived from it, namely the planar in 1897 and the unar in 1900 . In the first ten years, Zeiss only sold the lenses under the name Anastigmat , which was not protected and was also used by other manufacturers. Therefore, in 1900 the trademark Protar was introduced for the lens type . Carl Zeiss itself traced the development of one of the most important Zeiss lenses, the Tessar, back in 1902 to the Protar and the Unar.
The different series of the Protar each had a different fixed opening. The brightest Series I, for example, had a light intensity of 1: 4.5, the Series V a light intensity of 1:18.
At the beginning of the 20th century, Protare were considered to be "very good wide-angle lenses". Although they were manufactured well into the 1930s, they were not a particularly successful line of lenses. The Dagor developed two years later by CP Goerz was the first really successful anastigmat.
Lens optical background
An anastigmatic lens without field curvature must have a Petzval sum close to zero. The Petzval sum is the reciprocal of a sum of products that is formed from the focal length of the lenses used and their refractive index. So that a lens can even have a positive focal length, the sum of the individual focal lengths of the lenses used must also be positive. This means that the converging lenses dominate the optical system as a whole. If the lenses are all made of the same material, the Petzval sum can never reach zero. If, on the other hand, the converging lenses are made of a material with a particularly high refractive index, the Petzval sum and thus the field curvature are reduced. If the optical system also contains a diverging lens (negative contribution to the total focal length) made of glass with a low refractive index, the Petzval sum can approach zero.
proof
- ↑ a b Bernd K. Otto : Happy Birthday, Tessar! in: PhotoDeal, No. 37, 2/2002, pp. 32-37
- ^ A b Gregory Hallock Smith (2006) Camera Lenses: From Box Camera to Digital. SPIE Press. P. 140
- ↑ "Photographic Double Lens" Imperial Patent Office, Patent No. 56109, available online from the German Patent Office , accessed on May 25, 2011
- ↑ Zeiss original documentation from 1901, p. 30f; available online , accessed May 25, 2011
- ↑ Lexicon of all technology. Published by Otto Lueger. 2nd edition 1904–1920. P. 730
