Can Petri breech-lock lenses be converted or adapted to Canon EF, and where is flange distance measured?
Asked 10/4/2021
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I’m looking at inexpensive Petri breech-lock lenses and want to modify one for use on a Canon EF DSLR. I found references saying the Petri breech-lock flange focal distance is 43.5mm, which is only 0.5mm shorter than Canon EF’s 44mm. Where exactly is the flange focal distance measured from on the Petri breech-lock mount, and does that small difference make practical adaptation or conversion to EF possible without optical correction?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
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The flange focal distance is the distance from the mounting flange of the lens or camera to the film or image sensor plane. You can’t measure it on the lens, but you can measure it on the camera.
Petri breech mount lenses have a flange focal distance of 43.5mm. Yes, that is only 0.5mm less than the Canon EF 44mm flange focal distance, but there is no way to make an adapter unless you include focus corrective optics. It would also be very difficult to modify the lens mount as you would have to move the mount closer to the image sensor and still have a way to attach the lens to the camera.
Lenses with flange focal distances longer than 44mm can use a simple adapter. These include Petri Bayonet, Pentax M42, and Pentax K at 45.5mm, Olympus OM at 46mm, and Nikon F at 46.5mm.
Originally by user39427. Source · Licensed CC BY-SA 4.0
user39427
4y ago
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Generated from our catalog & community — verify before relying on it.
Flange focal distance is measured from the lens/camera mounting flange to the film or sensor plane. In practice, it’s defined by the camera mount side, not by some separate point deep inside the lens.
For Petri breech-lock, the flange focal distance is 43.5mm. Canon EF is 44mm. Because the Petri lens is designed to sit 0.5mm closer to the sensor than an EF camera allows, a simple mechanical adapter will not let it reach infinity focus.
So for EF, your options are:
- use an adapter with corrective optics, or
- do a destructive mount conversion that physically repositions the lens closer to the sensor.
That second option is difficult because you need to move the mount inward while still creating a secure EF attachment.
By contrast, lenses with a longer flange distance than EF can usually be adapted with a simple spacer-type adapter.
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