Archived posting to the Leica Users Group, 1996/09/13
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Leica has to measure the exact focal length of each M-lens because this
information is needed for producing the right slope for the barrel that
moves the little wheel of the camera's viewfinder coupling arm. Each lens
barrel is adjusted individually by hand to compensate the specifics of its
optics.
Gerard Captijn
Geneva, Switzerland.
>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
>
>
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