Chromatic aberration relates to the varying energy of the electrons that comprise the beam. Not all electrons generated at the gun assembly have the same energy. Electrons with different energies have different wavelengths. A magnetic field will have more of an influence on a longer wavelength electron. Thus, due to a variance in the energies of the electrons in the beam, it is not focused to a discrete focal point.

Diffraction is of most importance at the final probe forming lens. Diffraction is caused by the electrons wavelengths being out of phase. Thus, a lens will focus electrons of different phase to a different point depending upon the position of an electron in its wavelength when it passed through the lens. To correct diffraction, the electrons would have to be monochromatic and coherent. If these conditions were met, the lens could focus the stream of electrons to a point rather than to a disk of confusion.

Astigmatism occurs because of manufacturing imperfections within the electromagnetic lens. It is extremely difficult to manufacture an electromagnetic lens that forms a perfectly even magnetic field. Variances in the strength of the magnetic field around the circumference of the lens causes an elliptical focal spot. This problem can be overcome by segmenting the lens into many parts and being able to adjust the excitation to each part. In this manner, weak portions of the lens can be strenghtened.

Along with the electromagnetic lenses, metal aperature strips can be used to refine the beam. It is common for the modern SEM to be equipped with variable condensor and objective lens aperatures. These variable aperature strips will have a variety of pinhole sizes to choose from. It is the responsibility of the operator to choose the proper aperature size. To generalize, small objective aperature sizes will produce images with good resolution, good depth of field and minimal charging. This last statement requires two definitions. Depth of field refers to the ability to have a large change in specimen topography appear to remain in focus. SEMs allow for the specimen to be translated along the x, y and z axes. The z axis is defined as the distance from the final lens to the surface of the specimen. This distance is also referred to as the "working distance." Long working distances and small aperatures provide an image that appears to be in focus over a large change in z. Having dealt with depth of field, a brief statement on charging is in order. An ideal specimen would be conductive. If a less than ideal (non-conductive) specimen is attemted to be viewed, beam electrons can build up on the surface of the specimen, resulting in the phenomenon known as charging. Charging will be further explained in specimen-beam interaction/image formation.

After the objective aperature, the last manipulation of the beam prior to impingement upon the sample occurs in the final lens. The final lens is the heart of the SEM, and gives the instrument its name. Within the final lens are the raster coils. These coils raster or scan the focused electron beam over the surface of the specimen. Hen the name, scanning electron microscope. The raster coils scan the focused electron beam across the specimen much as one would read this page. You start at the top, read across the page to the end of the first line, drop down a line and back to the left and repeat. The scan of the raster coils is synchronized with the scan of the viewing screen. The raster coils are used to change magnification. To increase magnification, the coils can be made to scan a shorter line on the specimen. Since the size of the viewing CRT is fixed, the information generated by a shorter scan on the specimen must be enlarged to fill the viewing CRT. This is how magnification is changed.