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.
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.
