The Canon R lens mount is a physical standard for connecting a camera lens to a 35mm single-lens reflex camera body. It was introduced in March 1959 along with the Canon Canonflex.

The R mount was used on Canon's first single lens reflex (SLR) camera. The mount employed a "breech lock" system to attach the lens to the camera body.^[1]

The R mount was discontinued in 1964 and replaced with the Canon FL lens mount.^[2] Many mirrorless interchangeable-lens cameras are able to use Canon R lenses via an adapter.

Many of the lenses released for the Canon R mount have an automatic or semi-automatic diaphragm. These are distinguished by the name of the lens: "Super-Canomatic R" lenses are equipped with an automatic diaphragm, while "Canomatic R" lenses have a semi-automatic diaphragm, and a "Canon Lens R" has either a manual preset or manual diaphgram. With an automatic diaphragm, the view through the lens is maintained at the fully-open aperture while the photographer presets the aperture to the desired value; when the shutter is released, the lens stops down to the preset aperture during the exposure, then returns the aperture to fully open once the exposure is complete. The semi-automatic diaphragm works in the same way, with a small exception: the aperture does not return to fully open until after the photographer winds the film on following the exposure. Other lenses have a manual pre-set or fully manual diaphragm.^[3]

Super-Canomatic R lenses are equipped with two diaphragm control rings. The one closer to the front of the lens is the Preset Aperture Ring, used to preset the diaphragm setting for automatic opening and closing during exposure. The one closer to the back of the lens is the Visual Aperture Ring, which is used to preview depth of field. When the Preset ring is being used, the Visual ring should be turned to fully wide open. Likewise, when the Visual ring is stopped down, the Preset ring should be turned to fully open.^[4]: 5–7

To mount the lens, the breech-lock ring must be turned so the red dot is aligned with the positioning pin. The positioning pin fits into the notched lug at the 12 o'clock position on the camera body, then the breech-lock ring is turned clockwise (while facing the front of the lens) to secure it on the camera. For Super-Canomatic R lenses, if the charging lever is not pre-charged before mounting the lens on the body, the automatic diaphragm mechanism will not operate for the first exposure.^{[4]: 21–22}

Since the mechanism for controlling the aperture is different from both the later FL and FD mount, using R lenses on later bodies requires stopped-down metering mode and a manually-set aperture; however, the lugs, flange focal distance, and breech-lock ring are mutually compatible, so most R, FL, and FD lenses can use the same mount adaptors.

• 
  Three Canonflex cameras, showing optional waist-level magnifying finder (right), bellows (center), and clip-on external meter (left)
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  Canonflex R2000
• 
  Canonflex RP
• 
  Canonflex RM

• Canon Canonflex (1959)^[1]
• Canon Canonflex R2000 (1960)^[5]
• Canon Canonflex RP (1960)^[6]
• Canon Canonflex RM (1962)^[7]

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  Canon FL Lens 28mm (left) and Super-Canomatic Lens R 50mm (right), as seen from the front
• 
  Rear of Canon FL 28mm Lens (L) and Super-Canomatic Lens R 50mm (R)
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  Canon FL Lens 28mm (L) and Super-Canomatic Lens R 50mm (R)

Notes

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