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Adapter compatibility · Various → Canon

M42 to Canon EF adapter compatibility

Mounting a M42 (Pentax / Praktica screw mount) lens on a Canon EF body — the feasibility verdict, AF / IS / aperture-control / infinity-focus outcome, image-circle relationship, official and reputable third-party adapter SKUs, and the caveats worth knowing before you buy.

Verdict at a glance

Speed booster
MFIS lens-onlyAp. wheel

Mount specs

Lens side

M42 (Pentax / Praktica screw mount)

Flange distance
45.46 mm
Protocol
Mechanical only
Type
legacy-SLR

Body side

Canon EF

Flange distance
44 mm
Protocol
Canon EF
Type
DSLR

Flange-distance gap: only 1.46 mm (45.46 mm − 44 mm) — thinner than a rigid mechanical adapter can be built to reliably reach infinity focus. The verdict above recommends a focal reducer (Speed Booster) instead of a plain spacer.

Flange-distance schematic. Two rails share a sensor plane on the right. The Canon EF body register measures 44 millimetres; the M42 lens needs 45.46 millimetres. The orange region between their left edges is the 1.46 millimetre gap an adapter spans.Sensor planeCanon EF body · 44 mmM42 lens · 45.46 mm+1.46 mm adapter
Both distances right-aligned to the sensor. The gap is only 1.46 mm — thinner than a rigid adapter reliably builds to, so the verdict calls for a focal reducer instead of a plain spacer.

Adapter examples

  • Speed Booster / focal-reducer family

Caveats

  • 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.

Common questions

Will M42 (Pentax / Praktica screw mount) lenses autofocus on a Canon EF body through an adapter?
No — M42 → Canon EF adapters are mechanical only. Focus is fully manual; rely on the Canon EF body's focus peaking and magnify-to-focus aids to nail focus.
Does in-lens image stabilization (IS / VR / OS) still work through a M42 → Canon EF adapter?
Lens-side only — the M42 lens's IS / VR / OS unit operates, but it cannot synchronise with the Canon EF body's IBIS, so the dual-axis stabilisation native Canon EF lenses enjoy isn't available. Lens-side stabilisation still delivers most of the practical benefit.
What's the most-recommended M42 → Canon EF adapter?
No SKU in our curated catalogue covers M42 → Canon EF yet. Adapter examples photographers commonly use include the Speed Booster / focal-reducer family. Pair compatibility is mostly mechanical, so any well-built adapter at the correct flange distance should work — pick on build quality and tripod-foot integration.

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