How can i calculate the microns so i can be able to answer the diameter of high power field in microns?

equation





magnification of low power objective over the magnification of high power objective times the diamer of field under low power?|||look at this website http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artsep01/pjreducing.html