There are two electron optical aberrations which are commonly ignored
because they are unimportant, even ignored, in most electron micrographs.
These aberrations are radial distortion - also known as pincushion or barrel
distortion, depending on the sign - and spiral distortion, an effect which
never occurs in light optics. They are both illustrated in Figure 1. Since
these distortions are generally uninteresting, microscope manufacturers do
not provide substantial information about their instruments. Typically,
either there is no information at all, or a single number related to the
distortion constant is provided with no indication as to where in the field
of view it was measured or at what magnification, although both items are
needed to actually assess the distortion constant. Taking the most
optimistic view of the missing information, one can still determine that
radial distortion is a major factor in limiting the resolution attained by
Unwin and Henderson in their reconstruction of the two-dimensional structure
of bacteriorhodopsin.
A Two-Dimensional Nonlinear Nonlocal Feed-Forward Cochlear Model and Time Domain Computation of Multitone Interactions