EDIT:My intention was to destructively modify petri lens for use on EF DSLR.

I found some good Petri lens selling for less than 100RMB on the local auction site. I searched and found that petri breechlock is only 0.5mm shorter in flange than Canon EF so I would like to convert them. But after searching a lot I still don't know how exactly is the the petri mount's flange calculated. Would be gratified if somebody can tell me from where it is calculated.


1 Answer 1


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.

enter image description here

  • \$\begingroup\$ Maybe I have been unclear, but I was thinking of destructive conversion. Tearing out the breechlock and putting in the EF mount. So some measures have to be taken on the breechlock to ensure correct infinity focus. \$\endgroup\$
    – Jerryxhx
    Oct 5, 2021 at 2:02
  • \$\begingroup\$ I would measure the distance from the existing flange to a reference point somewhere else on the lens. Use that reference point measurement for a new EF mount that moves the lens elements 0.5mm closer to the sensor. \$\endgroup\$ Oct 5, 2021 at 2:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.