Anastigmat

The first Anastigmat was designed by Paul Rudolph for the German firm Carl Zeiss AG in 1890 and marketed as the Protar;[1]: 65–66(§103)  it consisted of four elements in two groups, as an asymmetric arrangement of two cemented achromatic lens doublets and was improved to a five-element, two-group design in 1891, substituting a cemented triplet for the rear group.

[1]: 67(§105) J H Dallmeyer Ltd first released a series of anastigmatic lenses consisting of multiple cemented achromats in 1895, designed by Hugh L. Aldis, marketed as the Stigmatic.

This in turn was improved by returning to the cemented rear group, resulting in the enduring four-element, three-group Tessar design (1902).

The so-called dialyte-type lens consists of a pair of air-spaced two-element achromats arranged back-to-back, and later was developed into the Goertz Artar by W.

The Schulz and Billerbeck company of Potsdam released Arbeit's modification as the Euryplan in 1903, generically known as the air-spaced Dagor.