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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