Helios's Biotar-derived M42 standard prime — the 44-2 58mm f/2, known for swirly bokeh, made in huge numbers.
The Helios 44-2 58mm f/2 was a Soviet standard prime in M42 screw mount, introduced in 1966 as the standard lens for Zenit 35mm SLRs. Its optical formula derives from the pre-war Carl Zeiss Biotar 58mm f/2, and it was manufactured in enormous quantities into the 1990s at several plants, including the Krasnogorsk Mechanical Plant (KMZ), the Minsk plant (MMZ) and the Jupiter plant in Valdai, making it one of the most-produced lenses in history.
It is a manual-focus M42 lens of 6 elements in 4 groups on the double-Gauss Biotar pattern, with an aperture range of f/2 to f/16 controlled by a two-ring preset mechanism rather than automatic stop-down. Minimum focus is 0.5 m and the filter thread is 49mm. Soviet specifications quoted light transmission of at least 0.82 and resolving power of 38 lines/mm centrally and 20 lines/mm at the edges.
The 44-2 is best known for its rendering: sharp centrally at f/2 with softer edges, moderate contrast, and background bokeh that swirls noticeably around the frame edges when there is textured detail behind the subject. That look has made it a favourite for portraits and video on adapted digital bodies, while the 58mm focal length also serves as a slightly long standard lens for street and general photography.
Supply on the used market is huge and prices are low, but build and glass quality vary between plants and production years, so buy on condition. Check for haze, fungus and cleaning marks on the soft front coating, stiff or dried helicoid grease, and a preset aperture ring that turns smoothly and closes the blades evenly. It adapts to any mirrorless system with a simple M42 adapter, which is how most examples are used today.