Different from the concept of depth of focus, the depth of field (Δu) in an electron microscope (EM) is the range of distance along the optical axis in which the specimen can move without the image appearing to lose the contrast and sharpness. That means, in the range of depth of field, the top and bottom of the specimen are simultaneously in focus. Therefore, the depth of field also limits the maximum thickness of the sample that can be observed by the microscope.
The depth of field, in principle, depends on the aperture size of the objective lens and the working distance. Figure 1988 shows schematics of the electron beam emerging from the final aperture in the objective lens and striking the specimen. A relatively large aperture and short working distance (W) in Figure 1988 (a) create a large convergence angle and thus a shallow depth-of-field. A small aperture diameter and long working distance (W) in Figure 1988 (b) create a small convergence angle and thus a large depth-of-field.
|Figure 1988. Schematics of the electron beam emerging from the final aperture in the objective lens and striking the specimen: (a) Small depth-of-field mode and (b) Large depth-of-field mode.