The optical formulas for lenses can`t be reversed. So the design of
optical devices is like a serch. The engineer is trying different element
combination untill he finds one wich fit`s the criterias.
First importend is the image quality, second to be close enough
to the desired data. So toleraces are inavoidable.
Are the differences always on the optimistic side? - Hm... competition?
Well acording to L.Hawkins web site, the Zuiko 2/21 could easily
be labeld as a 20mm F/1.8.
Frieder Faig
I don't know whether it is a current practice, but you used to be
able to unscrew the lens capsule from the mount on Leica lenses(since
the capsules were used in various mounts for different purposes).
There, scratched into the capsule, was the exact focal length of the
lens to a couple of decimal places as measured by the tech after
final assembly.
--
Winsor Crosby
Long Beach, California
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