Archived posting to the Leica Users Group, 1997/11/08

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Subject: Focussing 70-180 APO
From: leicas@isham-research.demon.co.uk (Phil Payne)
Date: Sat, 08 Nov 1997 19:05:56 GMT

In message <199711050458.XAA23683@s1-bngr.agate.net> Jack Milton writes:

> The exotic glasses used in apochromatic telephotos can expand and
> contract (at least this is how it was explained to me) causing infinity
> focus to vary according to temperature. It is nothing to worry about and
> usually in fast telephoto lenses you'll notice that there is some focus
> travel past the infinity mark possible to account for such variations.

Not just that - all kinds of thermal variations plus the possibility
of a minor knock affecting the infinity setting, etc.

Most serious professional lenses focus past infinity - but it's usual
for this also to be shown on the scales.  An indicated 'infinity'
_should_ be infinity - but the mount should go just past that point.

- --
Phil Payne