Archived posting to the Leica Users Group, 2003/05/26
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Rich Thank you for the reference link! I have used Leicas since 1946 an I still found almost all of his observances are spot on! I wish that all Leicaphiles would read all of that. Thanks again Jerry tripspud wrote: > Hi! > > I just run across something new (to me) from: > > http://208.218.135.74/photography/photo35.html > > and pasted below. Interesting, because I have the Canon 35/2 which > was used to work out the equation. Though other lenses work as well. > This is about unscrewing your screw mount lenses slightly, to gain more > close focusing. > > Cheers, > > Rich Lahrson > Berkeley, California > tripspud@transbay.net > > Breaking the 1 meter barrier. > > This only works for LTM lenses. The LTM thread is 26/inch or 0.9769 mm. (That's > an English Whitworth measure! Barnack > apparently wished to confound non-Leitz manufacturers would might make the > threads with a 1mm pitch and therefore never > quite fit the Leica correctly...) > > Anyway, using the thin lens equation, you can usually unscrew the lens by up to > 2 threads to focus a little closer. The equation: > > 1/Obj + 1/Imj = 1/Fl. > > Obj = distance between the lens and the object being imaged. > Imj = distance between the lens and the image. > Fl = Focal length of the lens. > > For example, my Canon 35mm f/2 lens focuses to 1 meter. How much closer can I > go? > Solve for the image distance: > 1/Img = 1/Fl - 1/Obj. > Substitute the real values: > 1/Img = 1/35 - 1/1000 = 0.02757... > so Img = 36.26. The lens only moves 1.27mm in its entire range! > Now we can find out how much closer unscrewing a thread gives us: > The Img distance will be 1.27mm + 0.977mm = 37.246mm > > Now solve for the object distance: > 1/Obj = 1/Fl - 1/Img. > Substitute the real values: > 1/Obj = 1/35 - 1/37.246 = 0.001723. > so Obj = 580mm, which really extends your range. > > -- > 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