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--------------------------------------------------------- By Bill Claff

In determining magnification I
presented the formula:

*m = x / f*

where *m* is
magnification, *x* is
extension from the image plane, and *f*
is focal length.

We can rearrange this formula
as:

*f = x / m*

So, if we know magnification for a given extension greater than zero we can
compute focal length.

For our initial example we have
this situation:

The extension is 1.72mm and the magnification is 1/30. When we apply the
formula we get a focal length of 51.6mm which is quite close to the
specification value of 51.61mm.

I use this technique with extension
tubes to determine the focal length of lens. Actual focal length is seldom the nominal
focal length and particularly when focused closer than infinity.

Knowing focal length at other than infinity focus is
particularly useful for close-up photographers who want to know what increased
magnification will result from a particular extension.

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