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

We can verify the stated
magnification with rays that run between the edges of the object and image
planes:

Note that the specified height of the image plane is the diameter of the image
circle.

Let's look more closely at the
focal point:

The distance between the focal
point (F') and the image plane (I) relates to magnification as follows:

*m = x / f*

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

This is how extension tubes increase magnification.

Note that x is zero at infinity and magnification is zero at infinity.