M42 Screw Mount (Universal Screw Mount / Pentax Thread)
Direct Answer
The M42 Screw Mount (also known as the Universal Screw Mount or Praktica/Pentax Thread, standardized under DIN 4503) is a standardized metric threaded lens interface featuring an M42 × 1.0 mm thread pitch and a 45.5 mm Flange Focal Distance. Popularized by Asahi Pentax Spotmatic and Soviet Helios-44 lenses, it is the most adapted vintage manual lens standard in mirrorless photography.
Source: DIN 4503 / Asahi Optical Co. Historical Standards / Zeiss Ikon Technical Specifications
Specs
| Thread Dimensions | M42 × 1.0 mm metric thread (42.0 mm diameter with 1.0 mm thread pitch) |
| Flange Focal Distance (FFD) | 45.50 mm (1.791 in) |
| Mounting Style | Direct screw-in thread (approx. 2 to 3 full revolutions to seat against flange) |
| Aperture Pin Mechanism | Center spring-loaded pin pressed by camera stop-down lever to close iris during exposure |
| Benchmark Cameras & Lenses | Pentax Spotmatic (Super-Takumar / SMC Takumar), Praktica L-series, Zenit-E (Helios-44-2 58mm f/2), Carl Zeiss Jena |
| Mirrorless Adaptability | Adapted seamlessly to Sony E, Canon RF, Nikon Z, L-mount, Fuji X, and MFT via inexpensive passive aluminum rings |
The Universal Thread Standard
First introduced on the East German Zeiss Ikon Contax S in 1949, the M42 × 1.0 mm screw mount became an open standard utilized by dozens of competing camera makers across Europe and Japan throughout the 1960s and 70s. Famous for vintage optical rendering (such as the swirly bokeh of the Soviet Helios-44-2 and the creamy multi-coated flare of Pentax Super-Takumars), M42 lenses remain intensely popular today on modern mirrorless cameras.