Petri Auxiliary Lenses Set

[theand]

Hello everyone from Italy, forgive my English!
I own a Petri 7S that is still working really great. It comes from my dad, and everytime I show him a photo I took with his old Petri he tells me the story of when it falled in a river upon the mount Etna in the 60s. I began to think that was a miracolous river and I'm thinking about taking there my old motorbike too. But this is another story.
Here comes the question. It's equipped with a 1:2.8 - 45mm objective. I've seen, on eBay, some so-called "auxiliary lenses", including a tele, a wide-angle and a range-finder. I feel very intrested in such lenses, to make my camera much more versatile. The problem is that it's impossible to find a set for my objective! There are some suitable for the 1:1.8 - 45mm, and others for the 1:2.8 - 4.5cm. And about this last objective, there's a seller specifying that it wouldn't fit a 1:2.8 - 45mm. My question is: WHY?? I'm not graduated in quantum physics but I always thought 4.5cm are 45mm, aren't they? What is the problem? Is anyone here able to explain me this 45mm curse? Did Petri change something in the objective and then marking them in different ways? My theory is that they passed from 49 to 52mm thread. Could anyone confirm this? I want those auxiliary lenses too!!:bang:
Thanks to all!

And

 

[sevo]

It might merely be a matter of the different thread sizes. But as many of the add-on lenses for fixed-lens rangefinders of that vintage don't only modify the view angle and magnification (as current tele and wide converters usually do), but also the real focal length, it might also be a matter of optical construction. On add-on lenses of that type, you have to scale focus on the add-on itself, and cannot use the rangefinder and main helical (as these are calibrated for the given focal length). The scale on attachments of that type is tied to the focal length and the distance between the lens nodal points and filter thread - and the latter will vary considerably depending on aperture and build, so these attachments are tied to the lens (or set of lenses) they were made for.

 

[btgc]

In case of your Petri I believe 4.5cm is truly on seller's mind, probably he recently sold older Petri which could have used 4.5cm marking. Cm thing were popular some decades before 7S were manufactured, at 70thies they marked focal length in mm. I just checked my Petri 7S and lens is marked 45mm not 4.5cm.

I'm not sure - probably aux lenses for 1.8 lens would suit 2.8 lens (though not opposite). Probably even any other lens set for 45mm focal length and with 52mm threads would match, but I am not sure. Thing you called rangefinder is hot shoe mountable viewfinder with framelines for "wide" and "tele" lenses. Be sure it's included in set otherwise you will have to guess framing.