Its story is a fascinating read, in the form of a longist Italian article. Translated by Google here.
The plot is full of twists:
Core based on the 1874 computation of the double-Gauss-type optical cell
Ideas drafted pre-WW-II in 1928, and again in 1937 by Kodak
Funded by the German Nazis in 1941 to guide weapons at night, 70mm f/1 produced
Project revived by NASA in 1966 to photography the moon in shadow (not satisfied w/ Angenieux 100mm f/1)
50mm f/0.7 project completed with 10 copies of the lens made, 6 sold to NASA, 1 kept by Zeiss
Other 3 lenses were bought by Stanley Kubrick who made a movie with scenes lit only by candlelight with it (Barry Lyndon)
The engineering looks eloquent to me. The key idea is to make a supersized 70mm f/1 that lights a much larger image circle, and then design a “condenser” that brute-forces your way to 50mm f/0.7 by shortening the focal length and condensing the light. Basically it’s adding a 0.7x teleconverter that gives 1 f-stop. (Boy, I really wish Nikon/Canon made these for FF lenses on crop bodies.)
In the 50mm f/0.7 optical design you can see the distinct pieces, the double-gauss (1-6) and the 0.7x condenser (7-8):
Examples of the double-gauss cell in the market are plentiful, here are a few, in order: Nikkor 50mm f/1.8, Leica 50mm Summarit-M and Zeiss 50mm f/2 Planar-ZM…
Focal lengths from 35mm to 100mm have been made as double-gauss by various manufacturers at various points in time. The quality of the double-gauss rests on glass types and manufacturing tolerances. Even though it’s an ancient design they can be some of the very best lenses you can buy (flatness of field is unrivaled with this design).