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The first Summicron, introduced in 1953, was a collapsible seven-element lens in Leica screw mount, catalogued SOOIC. Its performance rested on new rare-earth glasses: the earliest barrels used thorium, and lenses in roughly the 993,000 to 1,030,000 serial band used lanthanum LaK9, faintly radioactive and inclined to a warm cast that clears again under sustained daylight or ultraviolet. E39 filter thread, ten-blade diaphragm and one-metre focusing, in chrome over brass, on any screw-thread Leica.