|Deeply-curved doublets bracketing|
the iris are a double-Gauss hallmark.
Improved double-Gauss designs
introduce some mild asymmetry
image by Alf Sigaro (Image rights)
The cornerstone of "normal" lens designs in the f/1.7-f/2.0 class, the double Gauss formula originates with the Zeiss Planar of 1896. The classical version has six elements in a 1-2-2-1 configuration, symmetrical around the aperture stop.
Photographers may simply refer to a "Gauss" design lens. Strictly this is not correct, as Carl Friedrich Gauss had originally described a telescopic objective pairing a positive and a negative element. Using two Gaussian lenses back-to-back was the vital innovation for photography.
Countless derivatives of this formula have been manufactured with variations including air-spacing rather than cementing the inner doublets; splitting the outer elements into two, and so on. Lenses in the f/1.0–f/1.4 class are frequently these modified double-Gauss designs.
|This 7-element derivative achieves f/1.4|
(Olympus OM system Zuiko 50mm)
image by Voxphoto (Image rights)