Depth of field (DOF) is the distance along the optical axis either side of the focus point that remains in acceptable focus. DOF can normally be improved by closing the aperture on the objective, yet only at the expense of optical resolution and with a significant loss of light - not an acceptable method when using microscopes.
Without Depth of Field
With Depth of Field
Part of a computer chip seen under the SeeNano microscope. Adding depth of field provides significantly more details
allowing a better understanding of the structure with a 3 dimensional appearance (not oblique).