Double-Gauss lens

[1]: 245–248  Double Gauss/Planar tweaks formed the basis for most normal and near-normal prime lens designs with wide apertures for sixty years.

[2]: 118 Current double Gauss lenses can be traced back to an 1895 improved design, when Paul Rudolph of Carl Zeiss Jena thickened the interior negative menisci and converted to them to cemented doublets of two elements of equal refraction but differing dispersion for the Zeiss Planar design of 1896[4] to correct for chromatic aberration.

The main development was due to Taylor Hobson in the 1920s, resulting in the f/2.0 Opic and later the Speed Panchro designs, which were licensed to various other manufacturers.

In 1927, Lee modified the Opic design and increase the maximum aperture up to f/1.4, which was named the Ultra Panchro lens.

It was also the standard lens on the VEB Zeiss Ikon (Dresden) Contax S (1949, East Germany), the first pentaprism eye-level viewing 35mm SLR.

Several contemporaneous competing, but less famous lenses, were similar to the Biotar, such as Albrecht Tronnier's Xenon for Schneider Kreuznach (1925, Germany).

[2]: 122–123 [9] For example, three asymmetric Double Gauss lenses were produced in 1934 for Ihagee VP Exakta (1933, Germany) the type 127 roll film SLR camera: 8 cm f/2 versions of both the Biotar and Xenon, as well as the Dallmeyer Super Six 3 inch f/1.9 (UK).

By enlarging the rear group significantly (compared to a Double-Gauss type of more traditional focal length), the field of view was increased while keeping the aperture relatively large- making it, for a time, the fastest 28mm lens available for 35mm cameras by a large margin.

[34] The design is presently used in inexpensive-but-high-quality fast lenses such as the Sony FE 50mm f/1.8, the Canon EF 50mm f/1.8 and the Nikon 50 mm f/1.8D AF Nikkor.

Development of the Double Gauss
Zeiss Biotar 58mm f /2
Zeiss Biotar diagram