Elmar 90mm f/4

Max Berek's 90mm f/4 gave Barnack-era photographers reach without bulk: a slim, often collapsible tube that slipped into a coat pocket. It survived the screw-to-M transition (a collapsible M version arrived with the M3 era) and stayed in the catalogue into the late 1960s.

For newcomers: this is the cheapest way into vintage Leica telephoto — plentiful, tiny, and slow at f/4, with the gentle classic rendering that flatters portraits.