Adapter guide
Pentax K lenses on Canon RF
Yes, this works. Pentax K has a 45.46mm flange distance and Canon RF mount has 20mm, so a 25.46mm mechanical adapter with no glass fills the gap and infinity focus is preserved. Focus and aperture stay manual.
- Pentax K flange
- 45.46 mm
- Canon RF flange
- 20 mm
- Adapter thickness
- 25.46 mm
- Optical elements
- None required
Setting it up
Enable Release shutter w/o lens and turn on MF peaking. Canon APS-C bodies use a 1.6x crop rather than the 1.5x of other systems.
Automatic diaphragm. Pentax-A and later lenses add an A position on the ring that hands aperture control to the body; leave it off A on an adapter.
Pentax K lenses and their field of view on Canon RF
Effective focal length changes with sensor size. The lens itself does not change; the smaller sensor simply captures a narrower part of the image circle.
| Lens | Focal length | Aperture | Full frame | Frames like (APS-C (1.6x)) | Years | Used value |
|---|---|---|---|---|---|---|
| Pentax SMC Pentax 50mm f/1.2 | 50mm | f/1.2 | 50mm | 80mm | 1975-1990s | $300 to $800 |
| Pentax SMC Pentax-M 50mm f/1.4 | 50mm | f/1.4 | 50mm | 80mm | 1977-1984 | $60 to $210 |
Frequently asked
Do Pentax K lenses work on Canon RF cameras?
Yes. A plain Pentax K to Canon RF adapter works with no optics. Pentax K has a 45.46mm register and Canon RF mount has a 20mm register, so the adapter is a 25.46mm tube that puts the lens exactly where a Pentax K body would. Plain adapter ring. Because K keeps the 45.46mm M42 register, M42 lenses also mount on K bodies with a thin adapter and keep infinity.
Do Pentax K lenses focus to infinity on Canon RF?
Yes. Because the adapter restores the native 45.46mm register, infinity focus is preserved and the full focus scale on the lens stays accurate. Adapters machined slightly short let the lens focus past infinity by a small margin, which is harmless; adapters machined long stop short of infinity, so buy from a maker with tight tolerances.
What focal length does a Pentax K lens become on Canon RF?
On full frame Canon RF bodies (EOS R, RP, R3, R5, R6, R8) the focal length is unchanged: a 50mm lens frames as a 50mm. On APS-C (1.6x) Canon RF bodies (EOS R7, R10, R50, R100) a 50mm lens frames like a 80mm lens on full frame; the aperture and depth of field at a given distance do not change.
More on this mount: Pentax K mount reference