ABSTRACT
Aberrations arise from the use of spherical surfaces and the fact that refraction varies in each concentric annular zone of a surface. The aberrations may also vary in their effect with wavelength, as distinct from longitudinal chromatic aberration and transverse chromatic aberration, due to variation of refractive index with wavelength. The aberration performance of the complete lens can be investigated in more useful detail by the lens designer using other direct methods. Strictly speaking, a lens is aberration-corrected or optimized for one object conjugate distance, usually infinity, but closer distances are sometimes used. Spherical aberration varies with wavelength and gives a compound aberration termed spherochromatism, which can largely be corrected. Chromatic aberration occurs also if the electrons cease to be monochromatic due to fluctuations in the electromagnetic parameters determining the ‘focal length’ of the magnetic lens. Imaging systems which manipulate electron beams rather than rays of light also suffer from aberrations.
