Archived posting to the Leica Users Group, 2003/03/30
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Thermal expansion allowance it is. Angenieux and Zeiss film zooms have this too. I got caught by that factor way back in 19xx, and spoiled some football footage from the top of the press box in Tulane stadium. (That gives you a hint at when 19xx was.) Of course, if your lens sample goes TOO FAR past 8, you'll be able to focus on the back of your head. Sonny http://www.sonc.com - ----- Original Message ----- From: "Mark Rabiner" <mark@rabinergroup.com> To: <leica-users@mejac.palo-alto.ca.us> Sent: Sunday, March 30, 2003 4:38 AM Subject: Re: [Leica] 80-200 question > 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 - -- To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html