Will M42 lenses focus to infinity on a modern DSLR?

Depends on the body. On Pentax K bodies, yes — the Pentax-original M42-to-K adapter is a thin 1 mm spacer that preserves infinity focus. On Canon EF, yes through a Fotodiox / K&F adapter (44 mm vs 45.46 mm = 1.46 mm gap leaves room for a glass-less ring). On Nikon F (46.5 mm), no — the gap is negative, so you need a focal-reducing adapter with corrective glass that degrades image quality.

Are M42 Pentax / Praktica / Zeiss lenses worth using on a modern mirrorless body?

Often yes — the M42 ecosystem covers thousands of optically excellent primes: Pentax Super-Takumar (50 mm f/1.4, 28 mm f/3.5), Carl Zeiss Jena (Pancolar 50 mm f/1.8, Flektogon 35 mm f/2.4, Sonnar 135 mm f/3.5), Zenitar 16 mm f/2.8 fisheye, Helios 44-2 (58 mm f/2 swirly bokeh). On Sony E, Fujifilm X, or Canon RF, every M42 lens adapts cleanly with a $10-30 dumb mechanical ring.

Why is M42 sometimes called "Pentax screw mount"?

Pentax (Asahi Optical) standardised on M42 from the 1957 Asahi Pentax K through the entire Spotmatic SP / SP II / SP F / ES line until switching to the K bayonet in 1975. M42 itself was introduced by Carl Zeiss Jena in 1949 for the Contax S, but Pentax's mass-market adoption is what made M42 the de-facto SLR mount for almost every maker not named Nikon, Canon, Minolta, or Olympus through the 1960s and early 1970s.

Various · Legacy SLR mount

M42 (Pentax / Praktica screw mount) — flange distance, protocol, and adapter compatibility

42 × 1 mm screw-thread mount used by Pentax (Asahi Spotmatic era), Praktica, Zeiss, and many Soviet-bloc makes from the 1950s through 1980s. Pure mechanical interface — no AF, no electronic aperture coupling. Trivially adapted to every mirrorless mount and to Pentax K via a thin spacer. The lingua franca of cheap vintage glass.

Mount specifications

Flange focal distance: 45.46 mm
Throat diameter: 42 mm
Electronic protocol: Mechanical only (no electronic coupling)
Supported formats: full-frame, APS-C
Manufacturer: Various
Introduced: 1949 (discontinued)
Status: Discontinued

M42 (Pentax / Praktica screw mount) on the flange-distance axis

M42 (Pentax / Praktica screw mount) sits at 45.46 mm — highlighted in orange below. The flange-distance gap between the mirrorless and SLR clusters is the room a mechanical adapter occupies; that gap is why almost every SLR lens adapts onto every mirrorless body, and why the reverse is mechanically impossible.

Flange focal distance (mm)SLR / DSLRMirrorlessRangefinderMedium-format mirrorlessCinemaLegacy SLR

Adapting M42 (Pentax / Praktica screw mount) lenses onto other bodies

You own M42 (Pentax / Praktica screw mount) glass and want to mount it on a body with a different lens mount. Rows are sorted by feasibility.

Body mount	Result	Adapter examples	Caveats
Body mount Canon RF flange 20 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Canon RF body.
Body mount Canon FD flange 42 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Canon FD body.
Body mount Canon EF-M flange 18 mm	Mechanical MFno ISAp. ring1.5× crop	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Canon EF-M body.
Body mount Nikon Z flange 16 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Nikon Z body.
Body mount Sony E (incl. FE) flange 18 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Sony E (incl. FE) body.
Body mount Fujifilm X flange 17.7 mm	Mechanical MFno ISAp. ring1.5× crop	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Fujifilm X body.
Body mount Fujifilm GFX (G-mount) flange 26.7 mm	Mechanical MFno ISAp. ringvignettes	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Fujifilm GFX (G-mount) body.
Body mount Micro Four Thirds flange 19.25 mm	Mechanical MFno ISAp. ring2× crop	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Micro Four Thirds body.
Body mount L-Mount flange 20 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and L-Mount body.
Body mount Leica M flange 27.8 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Leica M body.
Body mount Canon RF (cine) flange 20 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Canon RF (cine) body.
Body mount C-mount flange 17.526 mm	Mechanical MFno ISAp. ring2.7× crop	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and C-mount body.
Body mount Konica AR flange 40.5 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between M42 (Pentax / Praktica screw mount) lens and Konica AR body.
Body mount Canon EF flange 44 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 1.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Canon EF (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Canon EF-S flange 44 mm	Speed booster MFIS lens-onlyAp. wheel1.5× crop	Speed Booster / focal-reducer family	Flange clearance is only 1.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Canon EF-S (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Nikon F flange 46.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -1.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Nikon F (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Sony A / Minolta A flange 44.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 1.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Sony A / Minolta A (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Pentax K flange 45.46 mm	Speed booster MFno ISAp. ring	Pentax K-mount M42 adapter (official thin spacer)	M42 flange 45.46 mm equals Pentax K flange 45.46 mm — Pentax's adapter is essentially a thin alignment ring. Infinity focus preserved exactly; no glass involved.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Pentax K (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount PL (Positive Lock) flange 52 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -6.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a PL (Positive Lock) (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Canon EF (cine) flange 44 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 1.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Canon EF (cine) (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Exakta flange 44.7 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 0.8 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Exakta (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount T-mount (T2) flange 55 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -9.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a T-mount (T2) (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Praktica B flange 44.4 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 1.1 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Praktica B (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Minolta SR / MC / MD flange 43.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 2.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Minolta SR / MC / MD (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Olympus OM flange 46 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -0.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Olympus OM (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.
Body mount Contax/Yashica (C/Y) flange 45.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -0.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.	Speed Booster equivalence calculator Plug in any M42 (Pentax / Praktica screw mount) lens and pick the focal-reducer family. The calc returns the effective focal length and aperture on a Contax/Yashica (C/Y) (APS-C), plus the full-frame equivalent angle of view after the body's crop stacks on top. Lens focal length (mm)Max aperture (f/N)Focal-reducer family Effective focal length 35.5 mm 50.0 mm × 0.71 on the reducer. Effective aperture f/1.28 0.99 stops brighter than f/1.80. Full-frame equiv. angle of view 53.3 mm 35.5 mm × 1.5× (APS-C sensor crop). A focal reducer concentrates the lens's image circle, so both focal length and f-number scale by the same ratio (stops gained = `−2 · log₂(ratio)`). The body's crop factor still applies on top — the full-frame-equivalent angle-of-view multiplier is ratio × body crop. Calculator is informational; verify against the adapter vendor's per-lens compatibility chart before purchase.

Adapting other lenses onto a M42 (Pentax / Praktica screw mount) body

You own a M42 (Pentax / Praktica screw mount) body and want to mount glass from other systems. Mirrorless-lens-onto-DSLR-body combinations are omitted (rear element collides with the mirror box).

Lens mount	Result	Adapter examples	Caveats
Lens mount PL (Positive Lock) flange 52 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between PL (Positive Lock) lens and M42 (Pentax / Praktica screw mount) body.
Lens mount T-mount (T2) flange 55 mm	Mechanical MFno ISAp. ring	generic mechanical adapter ring (multiple vendors)	Mechanical adapter only — no electronic communication between T-mount (T2) lens and M42 (Pentax / Praktica screw mount) body.
Lens mount Canon EF flange 44 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -1.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Canon EF-S flange 44 mm	Speed booster MFIS lens-onlyAp. wheelvignettes	Speed Booster / focal-reducer family	Flange clearance is only -1.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Canon FD flange 42 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -3.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Nikon F flange 46.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 1.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Sony A / Minolta A flange 44.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -1.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Pentax K flange 45.46 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 0.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Canon EF (cine) flange 44 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -1.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Exakta flange 44.7 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -0.8 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Praktica B flange 44.4 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -1.1 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Konica AR flange 40.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -5.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Minolta SR / MC / MD flange 43.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only -2.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Olympus OM flange 46 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 0.5 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.
Lens mount Contax/Yashica (C/Y) flange 45.5 mm	Speed booster MFIS lens-onlyAp. wheel	Speed Booster / focal-reducer family	Flange clearance is only 0.0 mm — a plain mechanical adapter cannot reach infinity focus; a focal reducer (Speed Booster) with optical glass is required. Speed Boosters typically widen the effective focal length (~0.71×) and add ~1 stop of light, which can be desirable on crop bodies.

Adapter SKU teardown

Curated adapter SKUs that involve the M42 (Pentax / Praktica screw mount) mount on either side, with the operational specifics — body-side electronic contact count, firmware-update path, weather sealing, and whether optical glass is in the path.

K&F Concept M42-NEX Pro
released 2018
M42 (Pentax / Praktica screw mount) lens → Sony E (incl. FE) body
- no firmware updates
- not weather sealed
- no glass — pass-through
- Pro-line M42 (42 mm × 1 mm screw) onto Sony E with a copper-inset bayonet for tighter mechanical fit and an anti-reflective matte-black interior to suppress flare from bright legacy elements.
- Mechanical-only — no electronics, no AF, no aperture coupling. M42 aperture is set on the lens's own ring (or via its stop-down lever, depending on the lens). Infinity-focus calibrated; works with Carl Zeiss Jena Pancolar / Flektogon, Pentacon, Helios-44, Super-Takumar, Industar, Mir, and every other M42 screw-mount lens shipped between 1949 and the early 1980s.
K&F Concept M42-EOS R Pro
released 2020
M42 (Pentax / Praktica screw mount) lens → Canon RF body
- no firmware updates
- not weather sealed
- no glass — pass-through
- Pro-line M42 screw-mount onto Canon RF — Canon's 20 mm flange (vs Canon EF's 44 mm) leaves plenty of clearance for the adapter's body, and infinity calibrates cleanly on every R-line body (R5 / R6 / R6 II / R3 / R8 / R10 / R50 / R100).
- Mechanical-only — no electronics, no AF. Aperture is controlled by the M42 lens's own ring or stop-down lever. RF body's focus magnification and peaking aids handle manual focus.
Urth M42 to Sony E
released 2019
M42 (Pentax / Praktica screw mount) lens → Sony E (incl. FE) body
- no firmware updates
- not weather sealed
- no glass — pass-through
- Recycled-aluminum body with anodised matte-black finish and plastic-free packaging; one tree planted per product via Eden Reforestation Projects or One Tree Planted.
- Mechanically equivalent to K&F's Pro line internals — same OEM-grade Shenzhen build — at roughly 80-150% the price, differentiated on sustainability commitment and finish quality.
Kipon Baveyes M42-MFT 0.7x
released 2016
M42 (Pentax / Praktica screw mount) lens → Micro Four Thirds body
- no firmware updates
- not weather sealed
- optical glass in path (focal reducer)
- 0.7× focal reducer (Caldwell-licensed optics) for M42 screw-mount lenses onto Micro Four Thirds bodies (OM-1 II / OM-5 / G9 II / GH7 / Pen E-P7). ~1 stop wider effective aperture and reduced 2× crop factor (effective ~1.4×).
- Purely optical — no electronics, no AF, no firmware. M42 lens aperture handled by the lens's own ring or stop-down lever. Lets vintage M42 wides (Helios 44-2 58 mm f/2, Mir-1 37 mm f/2.8, Carl Zeiss Jena Flektogon 35 mm f/2.4) regain something closer to their original field-of-view on MFT.
K&F Concept M42-NZ
released 2019
M42 (Pentax / Praktica screw mount) lens → Nikon Z body
- no firmware updates
- not weather sealed
- no glass — pass-through
- M42 (Pentax / Praktica 42 mm screw) lenses onto Nikon Z. Mechanical ring — no AF, no electronics. Most M42 lenses use an Auto/Manual switch (set to M) or a rear stop-down pin the flat adapter clears, so the aperture ring controls the diaphragm.
- M42's 45.46 mm register clears Nikon Z's 16 mm by 29.46 mm — the deepest mirrorless gap, so infinity is the most forgiving of any Z adapt. Fits Takumar, Helios, Industar, Jupiter and the rest of the vast M42 universe.

References

https://en.wikipedia.org/wiki/M42_lens_mount

Common questions

Will M42 lenses focus to infinity on a modern DSLR?: Depends on the body. On Pentax K bodies, yes — the Pentax-original M42-to-K adapter is a thin 1 mm spacer that preserves infinity focus. On Canon EF, yes through a Fotodiox / K&F adapter (44 mm vs 45.46 mm = 1.46 mm gap leaves room for a glass-less ring). On Nikon F (46.5 mm), no — the gap is negative, so you need a focal-reducing adapter with corrective glass that degrades image quality.
Are M42 Pentax / Praktica / Zeiss lenses worth using on a modern mirrorless body?: Often yes — the M42 ecosystem covers thousands of optically excellent primes: Pentax Super-Takumar (50 mm f/1.4, 28 mm f/3.5), Carl Zeiss Jena (Pancolar 50 mm f/1.8, Flektogon 35 mm f/2.4, Sonnar 135 mm f/3.5), Zenitar 16 mm f/2.8 fisheye, Helios 44-2 (58 mm f/2 swirly bokeh). On Sony E, Fujifilm X, or Canon RF, every M42 lens adapts cleanly with a $10-30 dumb mechanical ring.
Why is M42 sometimes called "Pentax screw mount"?: Pentax (Asahi Optical) standardised on M42 from the 1957 Asahi Pentax K through the entire Spotmatic SP / SP II / SP F / ES line until switching to the K bayonet in 1975. M42 itself was introduced by Carl Zeiss Jena in 1949 for the Contax S, but Pentax's mass-market adoption is what made M42 the de-facto SLR mount for almost every maker not named Nikon, Canon, Minolta, or Olympus through the 1960s and early 1970s.

Mount specifications

M42 (Pentax / Praktica screw mount) on the flange-distance axis

Adapting M42 (Pentax / Praktica screw mount) lenses onto other bodies

Adapting other lenses onto a M42 (Pentax / Praktica screw mount) body

Adapter SKU teardown

K&F Concept M42-NEX Pro

K&F Concept M42-EOS R Pro

Urth M42 to Sony E

Kipon Baveyes M42-MFT 0.7x

K&F Concept M42-NZ

References

Common questions

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