Physical Principles of Electron Microscopy: An Introduction to TEM ...

Electron Optics 55
point from the optic axis. This spiral distortion was illustrated in Fig. 2-2c.
Again, compensation is possible in a multi-lens system.
For most purposes, distortion is a less serious lens defect than aberration,
because it does not result in a loss of image detail. In fact, it may not be
noticeable unless the microscope specimen contains straight-line features. In
some TEMs, distortion is observed when the final (projector) lens is
operated at reduced current (therefore large Cs) to achieve a low overall
magnification.
Curvature of field is not a serious problem in the TEM or SEM, because
the angular deviation of electrons from the optic axis is small. This results in
a large depth of focus (the image remains acceptably sharp as the plane of
viewing is moved along the optic axis) as we will discuss in Chapter 3.