Problems with lenses: aberrations
Over the last 300 years, glass lenses have developed to near perfection, while electromagnetic lenses remain quite imperfect. There are at least 10 kinds of defects for electromagnetic lenses but we will emphasize the ones that limit microscope performance in substantial ways.
Spherical and chromatic aberrations limit the resolution of conventional electron microscopes. Both these defects are unavoidable when using static rotationally symmetric electromagnetic fields. It is necessary to learn how to use the microscope to minimise them.
Spherical aberration is the type that is most significant in defining the performance of the objective lens.
Chromatic aberration worsens for thicker samples. To reduce this problem it is good to make thin samples (e.g. thin foils for physical science preparation).
Astigmatism affects the ability to focus an image but is totally correctable.