After creating the first Anastigmat for Zeiss in 1889 Paul Rudolph stayed at the company for a while, designing a further fine lens architecture: The 6-element Planar. It was also anastigmatic and got its name for its image plane which was almost plane and not spherical. Therefore it was made of six elements, two converging lens elements, one strongly diverging element, the aperture, another strongly diverging element and again two converging elements. The first version was a symmetrical design with lens speed f4.5. It was derived from the Double-Gauss lens. That basic lens concept had been developed by US-American telescope maker Alvan Clark in 1888 who just put two lenses of Carl Friedrich Gauss' Fraunhofer telescope lens concept (3 lens elements: meniscus plus convex plus concave) together, arranging them back to back, a concept still used today, for example for 50mm f1.8 standard 35mm camera lenses.
The strengthes of the Planar's construction could be further developed after the invention of lens coating because the original construction suffered flare. A variant for 35mm film with seven elements was made (Glatzel-variant). Nowadays the Planar is still a Zeiss product. And it gave the model for many other lens constructions. And not without reason: The Carl Zeiss Planar 1:0,7/50 mm is the fastest available photography lens. This super-fast variant was only used by the NASA for espionage tasks and by Stanley Kubrick to film scenes with just candle light. Almost all very fast lenses with focal lengthes between 35 and 85 mm seem to be derived from the Planar.
