The depth of field, D_ob is the range of distance along the optical axis in which the specimen can move without the image apppearing to lose sharpness. This obviously depends on the resolution of the microscope.

The depth of focus, D_im is the extent of the region around the image plane in which the image will appear to be sharp. This depends on magnification, M_T.

Both depth of field and depth of focus are strongly dependent on changes in aperture (hence the semiangle b) and working distance (d_ob).

What is the depth of focus at a magnification of ×20000 for an electron microscope with b = 1degree, d_ob = 1 mm?