Originally Posted by
Arne Croell
The basic equation for calculating the distance b between the film plane and the rear nodal point of the lens, which is approximately your bellows extension, is:
b=f·(1+m)
where f is you focal length and m is your magnification, i.e. for an image that is half life size, m=1:2=0.5. So for 1:2 you get 270mm extension, for 1:1 twice the focal length as mentioned by Christopher, for 2:1 it is 3x the focal length, i.e. 540mm in your case, etc.
As an aside, the distance a between the object and the front nodal plane is
a=f·(1+1/m)
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