Relationship of depth of focus to depth of field?

I have found only one set of equations on the web relating depth of
focus (dfocus) to depth of field (dfield) and am after a reference from
a text book. Can anyone recommend a reference and page number? The
equations are from

http://nsm1.fullerton.edu/~skarl/EM/Microscopy/

and go like this:

dfocus = M * M * R / NA

where M = magnification, R = resolution, NA = numerical aperture

and

R = L * 0.61 / N.A

where L is lamba, i.e. wavelength of light. Also

dfield = L / N.A * N.A

therefore

-----> dfocus = M * M * dfield * 0.61 <------

.

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