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The equations for resolution in z are different for a plane reflector, a
thin fluorescent lamina and a subresolution bead. The commercial story on
this is a mess of unattributed equations of obscure provenance. You may find
useful information in the following article, which is a permitted preprint
of an article about confocal microscopy to be edited and published in the
Elsevier online 'Comprehensive Biophysics', where Tony Wilson, Gail
McConnell and I have tried to make sense of it all :
http://www2.mrc-lmb.cam.ac.uk/images/groupleaders/Confocal_microscopy_Amos_McConnell_Wilson.pdf