The "f number" of the SEM is about 200, so it's depth of field is much larger than an optical microscope or camera. Also, the SEM signal of bright and dark area

The "f number" of the SEM is about 200, so it's depth of field is much larger than an optical microscope or camera. Also, the SEM signal of bright and dark areas is made of electrons, which can travel in curved paths and around corners. This gives the SEM image an appearance that is distinct from light images, and is helpful when viewing 3d objects for ease of feature recognition as well as aesthetic quality. Even at low magnification, SEM is often more useful. Thanks!