Archived posting to the Leica Users Group, 2003/03/30
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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 - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html