[Leica] Unscrewing your screw mount lens to get closer...

[tripspud <tripspud@transbay.net>]

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.


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