Manual Focus Forum

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.

(an Tessar for example has almost duble price if it is on M42 than the same lens on B bayonet)

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 £5.

Interesting note: The PB 2.4/50 pancake lens in bits on my desk is blatantly based on the M42 Tessar mechanically, even if it's different optically.

Rob.

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

montecarlo wrote:

About the flange distance I remembered that I saw once on a site that it would be the same as M42. So, that information must be wrong or I don't remember correctly.

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.

Thank you all for the informations about the issue

Seele wrote:

... 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.

...Yes thats why I watch that the lens to have the A/M swich or only manual lenses (I do not want to modify the only auto lenses). About the adapter, until I was aware about de flange distance I was happy seeing so much offers of M42->K Pentax adapter, but most of the do not allow the infinity focus only a few (the original Pentax, and HAMA- which I do have, KALT, etc...) but hopefuly I got in in time an finally I bought a HAMA adapter.

Thank you again and all the best.