1. Determine the magnification. This magnification (M) is the focal length of the lens (Fl) divided by the focal length of the lens minus the distance to the object (FlD) The equation
http://www.wikihow.com/CalculateMagnification

you multiply the magnification of the lense (usually x10) by the magnification of the settting.
http://wiki.answers.com/Q/How_is_the_total_magnifi...

Magnification is equal to the focal length of the object divided
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