Re: numbers on Summicron focusing ring, again

[dlevy@worldy.com]

At 08:58  13/09/96 -0400, you wrote:
>
>Thanks to those who explained the "22" on my Summicron focusing ring.  
>Now, my next question.  My black 50mm Summicron, which is #3,618,xxx, is 
>engraved with a "22," but in Eastland's _Leica M Compendium_ there is a 
>photo of a chrome specimen serial number 3,606,xxx which is engraved 
>"19."  
>
>Why would the actual focal lengths of these two lenses be different when 
>they were presumably manufactured within a couple of years of each other? 
>Wouldn't this need to be due to a change in the design of the lens?? 
>
>Thanks for any input on this,
>
>Tom Knoles
>tgk@mwa.org


Tom:
Lens manufacturing because of its very nature creates variances in its
tolerances. We generally think of these tolerances in terms of performance,
such as flare control (read the threads on this discussion here), light fall
off, etc. I have seen a couple of reviews which have measured the actual vs.
stated focal length of a lens, but it was several years ago in science
oriented mags (if memory serves me correctly). The variance is partially a
result of the hand grinding and polishing of the lens and the tolorances of
the machining of the barrel, etc. While, I have not discussed with others
the reason Leica inscribed this variance, I would venture a guess it was so
that it may have been used to match viewfinders to specific lenses (I'm
talking external finders) and thereafter so the user could adjust (mentally)
the inscribed brightline finder to the actual lens focal length. 

While this variance is small in the scheme of things, its not of concern to
the slr user, who sees the actual image (in reality from 90% up) in the
finder, and possibly only Contax of past years would have or could cared
enough about technical excellence to have also done it on their rangefinder
lenses (which they didn't).
Brian Levy, J.D.
Toronto, Ont.
dlevy@worldy.com