Hello Jean-Pierre!
A complete explanation is beyond my possibilities. As for my English...
In fact, there are several problems. All are due to the fact that Sonnars were originally made for Contax lenses, not Leica type ones.
For example, Leica rangefinders are calibrated for 51,6mm of focal length; Contax ones for 52,4mm. Brian Sweeney proposes a method to correct that:
http://ussrphoto.com/Wiki/default.asp?WikiCatID=50&ParentID=49&ContentID=1570&Item=Jupiter%2D3+Clean%2C+Lube%2C+AdjustAbout prewar Feds, it's more troublesome, as bodies and lenses were appaired one by one: they did not have the Leica specifications. So, if I want to mount a Sonnar on a Fed S, first I must control the register (28,8mm) and correct it if necessary.
All these questions are even more evident with a Sonnar 2/8,5cm. With a 2/5cm, you can use the diaphragm to be "within". With a 2/8,5cm, it's more difficult with the reduced depth of field. If you add that the origin of the Sonnars we have now is very difficult to trace...
The result: some lenses are completely unusable without adjustment. And you are very lucky if you find a 2/8,5cm Sonnar which covers correctly the whole distance range. Mine (a 1951 Jup 9) is perfect at 1,15m, but gets infinity only from f4/5,6...
If you want more, please phone!
Amitiés. Jacques.
