Archived posting to the Leica Users Group, 2003/03/30

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Subject: Re: [Leica] 80-200 question
From: Mark Rabiner <mark@rabinergroup.com>
Date: Sun, 30 Mar 2003 02:38:21 -0800
References: <E20A2B6E-628E-11D7-9BBA-000393802534@mac.com>

Martin Howard wrote:
> 
> I'm no optical expert, but AFAIK, the "focus beyond infinity" has to do
> with heat expansion in telephoto lenses.  If I recall correctly, heat
> causes the lens elements to shift ever so slightly (because of the
> metal in the barrel?) increasing the distance between the film plane
> and lens elemenst (same as focussing closer than infinity does).  In
> normal lenses and short telephotos, this is not an issue, but in longer
> (and wider) teles, it means that "infinity" focus can be difficult to
> obtain (after all, "infinity" in lens design is only something like
> 1000x the focal length), so high quality, long telephoto SLR lenses can
> be focussed "beyond" infinity to allow for this.
> 
> Since this is an SLR, you focus by sight (as opposed to a rangefinder),
> which means that if things look right in the viewfinder, they'll be
> fine on film.
> 
> More knowledgable people will hopefully fill in the details/add
> corrections.  A suitable net search will probably also help.
> 
> M.
> 
I bet you're right Martin, I thought it might be cold or filters but no
it's heat and you're right!


Mark Rabiner
Portland, Oregon USA
http://www.rabinergroup.com
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Replies: Reply from "\(SonC\) Sonny Carter" <sonc@sonc.com> (Re: [Leica] 80-200 question)
Reply from "Gary Williams" <nasmformyzombie@mindspring.com> ([Leica] R8)
In reply to: Message from Martin Howard <mvhoward@mac.com> (Re: [Leica] 80-200 question)