Dagor

The Dagor is a camera lens that was patented by Emil von Höegh in 1892 for the optical institute CP Goerz , Berlin. It was one of the most successful photographic lenses in the late 19th century . The Dagor is a double anastigmat (Dagor = D oppel- A nastigmat- Go e r z).

Lens optical historical background

Until well into the end of the 19th century, it was impossible to construct a camera lens that had no significant astigmatism and, at the same time, no strongly curved field of view . A flat field of view without astigmatism is only obtained if the Petzval sum of the lens is close to zero.

The Petzval sum is calculated as the reciprocal of a sum of products that are 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. In other words, the convergent lenses must overall dominate the optical system. 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.

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. Barium-containing crown glass (e.g. BaK4) has a significantly higher refractive index than "normal" crown glass. The only possibility that existed before to greatly reduce the curvature of field was to accept blurring astigmatism.

Lens scheme of Dagor

Based on the new barium crown glasses, Paul Rudolph designed the four-lens Protar for Carl Zeiss as the first real anastigmate . While Rudolph was working on a landscape lens patented in 1893 , the set anastigmat of series VI and VIa, from 1891 , Emil von Hoegh competed for the Goerz Optical Institute and Hugo Adolph Steinheil for CA Steinheil & Sons to develop a double- Anastigmates. The patent was granted to Goerz in 1892.

Although the Protare were manufactured by Zeiss into the 1930s, they were not a particularly successful line of lenses. The Dagor, on the other hand, was the first really successful anastigmat. The license for Great Britain was awarded in 1893 to the company Ross (company) in London, that for Austria-Hungary to Karl Fritsch in Vienna.

The Plasmat is a further development of the Dagor .

construction

The Dagor consists of two symmetrical lens groups. Each of these two lens groups in turn consists of three individual lenses cemented together (triplet). The diaphragm is located between the triplets . The triplets are each corrected for longitudinal chromatic aberration , spherical aberration , astigmatism and field curvature . The symmetrical construction also eliminates lateral chromatic aberration and coma and ensures a distortion-free image.

Individual evidence

1. ^ Gregory Hallock Smith: Camera Lenses. From Box Camera to Digital. Spie Press, 2006, pp. 139f.
2. ↑ Photographic double lens. Imperial Patent Office, Patent No. 56109, available online from the German Patent Office , accessed on May 25, 2011.
3. ^ Encyclopedia Britannica. Volume 21. P. 571ff.
4. ^ ^{A b} Gregory Hallock Smith: Camera Lenses. From Box Camera to Digital. Spie Press, 2006, p. 140