The 1930s collapsible Leitz 50mm f/2 — the Summicron line's screwmount ancestor.
The Leitz Summar 5cm f/2 was introduced in 1933 as the first f/2 standard lens for the screw-mount Leica, ahead of the Hektor f/2.5 it outclassed. Most were collapsible chrome lenses, with early rigid and black-nickel versions made in small numbers, and the Summitar replaced it from 1939.
It is a six-element, four-group double-Gauss design in Leica Thread Mount, focusing to 1 metre and stopping down from f/2 to f/12.5 on early continental scales. The uncoated optics sit in a collapsible chrome barrel of about 200g using A36 clamp-on or SOOMP hood accessories, and apertures are set by a small front ring without click stops.
Wide open it renders with low contrast, notable field curvature and a swirling background that collectors now seek out for portraiture, sharpening usefully by f/4. Its soft front glass marks it against the later coated Summitar and Summicron, so clean glass is the whole game on this lens.
Cleaning scratches and haze on the soft front element are the rule rather than the exception, so inspect glass before price. Check the collapsible barrel locks firmly and the aperture ring turns without play. Rare variants such as rigid, black nickel or Tropen bodies trade at strong multiples of the common chrome lens.