PB flange distance is about 1mm less than M42, to allow for the thickness of the Praktica M42->PB adapter which has electronic contacts. All PB lenses I have or know of have a separate mount attached with 4 screws, so you could remove the mount & aperture lever and attach an M42/K mount chopped off the back of another lens (as with Contax method linked here: http://oomz.net/mf/viewtopic.php?id=4374), but you'd still lose infinity focus.

Not sure where you are but round here the 2.8/50 Tessar in PB mount is rare and expensive (apparentlt there were'ne many produced - the standard lens was the remounted Pentacon 1.8/50), whereas M42 Tessars from the old L-series Prakticas are all over the place and can be had for about

Last edited by BC1 (2007-01-25 18:41:11)

The flange distance (as in "registration") of the Praktica B-mount is actually slightly less than that of the M42-mount.

When Praktica developed the B-mount, it was necessary to make sure that it has a certain amount of backward-compatibility. So, the camera body was made to be a little thinner than the M42 cameras, so that an adapter can be fitted between the camera and the M42 lens.

In contrast to Pentax's approach when making the transition to M42 to the K-mount, Praktica's approach makes a lot of sense. The K-mount camera retains the same registration but with a wider throat, so the adapter sits inside the lens throat, away from the normal lens locking mechanism, and requires its own, which can only be disengaged by poking it with a ball point pen (Pentax's own instructions). In that sense, the adapter pretty much stays on the camera, making mix-and-match of original K-mount and M42-mount lenses a royal pain. In addition to that, the Pentax adapter does not include an inner flange which forces the stop-down actuator pin in, to make sure that the automatic diaphragm mechanism of the lens is truly disengaged; I do believe that there's been plenty of pictures spoilt by not having the lens set to manual operation.

The Praktica adapter solves all these problems all at once: it fits outside the camera bayonet mount, thus using the camera's own lens locking device for holding it in position. It also incorporates the inner flange so as to prevent the lens from not stopping down, and to force the metering mechanism to work at stop-down mode, it has an "outrigger" which contains some electronic components to inform the exposure meter. So, if you wish, you can fit individual adapters to each M42 lens and work normally with the only loss as automatic diaphragm.