Canon RF vs. M42 Screw Mount
Comparing 2 standards across 12 specifications. 12 key differences identified.
The Canon RF Mount (introduced in 2018 for the EOS R system) is a 3-tab full-frame mirrorless bayonet mount featuring a 20.0 mm Flange Focal Distance, a wide 54.0 mm inner throat diameter, and an advanced 12-pin high-speed data bus. Retaining the large 54mm diameter of the classical EF mount, it enables ultra-fast aperture lenses such as the RF 28-70mm f/2L and 50mm f/1.2L.
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.
Specification Comparison Table
12 Parameters| Parameter / Spec | ||
|---|---|---|
|
Flange Focal Distance (FFD)
|
20.00 mm (0.787 in) | 45.50 mm (1.791 in) |
|
Inner Throat Diameter
|
54.00 mm (2.126 in — identical to legacy Canon EF mount) | — |
|
Bayonet Tabs
|
3 asymmetrical stainless steel bayonet tabs with clockwise mounting twist | — |
|
Electronic Interface
|
12 contact pins (high-bandwidth bidirectional bus for lens control ring and Coordinated IS up to 8 stops) | — |
|
Legacy Adaptability
|
100% native AF/IS compatibility with all Canon EF/EF-S lenses via Canon EF-EOS R adapter (24mm spacer) | — |
|
Lens Control Ring
|
Physical programmable control ring on all native RF lenses (aperture, ISO, exposure compensation) | — |
|
Camera Systems
|
Canon EOS R5 / R6 / R3 / R8 / R, Cinema EOS C70 / C80 / C400, APS-C EOS R7 / R10 | — |
|
Thread Dimensions
|
— | M42 × 1.0 mm metric thread (42.0 mm diameter with 1.0 mm thread pitch) |
|
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 |