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

[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]

Subject: Re: [Leica] 80-200 question
From: Martin Howard <mvhoward@mac.com>
Date: Sun, 30 Mar 2003 01:06:24 -0800

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.

- --
To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html

Replies: Reply from Mark Rabiner <mark@rabinergroup.com> (Re: [Leica] 80-200 question)