Bertele had already made the Ernostar at Ernemann — a fast lens with too many air–glass faces, so it flared. At Zeiss he glued more of those faces together. The name is from Sonne, the sun: a 50 mm that opened to f/1.5 on the new Contax, when coatings did not yet exist and every extra surface stole light.
This board is the seven-glass recipe: a front singlet, a thick cemented triplet, then another behind the stop. Rudolph’s Planar is the other Zeiss anastigmat of the era — more air, more surfaces, a flatter field. The Sonnar is the compact, contrasty one. In the USSR they called the copies Jupiter.