On the basis of principle of the Depth of Field (DOF), the relationship between the DOF of microscopic system with its geometric DOF and physical DOF was researched. A new view that the relationship between the geometric DOF and the physical DOF satisfies “OR” instead of “SUM” in mathematics was proposed. Based on the view, this paper deduced the DOF formulae for the microscope equipped with an electronic ocular. To verify the derived results, a capillary measurement system was designed. With the measurement system, the DOFs for a microscope with a transversal magnification of 10×20 was measured experimentally by varying its effective numerical apertures and those for different types of microscopes were also measured when the effective numerical apertures were equal to the nominal values of the objectives. Obtained results show that the measured DOFs are in good consistent with the data calculated by the deduced formula. The theoretical analysis and measured results both indicate that the DOFs for the microscope system should equal the larger value between the physical DOF and the geometric DOF rather than the sum of the two values.