Once in a while we have the privilege of participating in an activity that promises to radically improve our technical capability to explore materials at the microscopic level. The recent practical demonstration of aberration correction in the SEM, the TEM and the STEM signify nothing less than a revolution in the way that we design and operate electron optics for microscopy.

Scherzer understood from the beginning of electron microscopy that an open magnetic lens having cylindrical symmetry will have positive, non-zero aberration coefficients, and so therefore will always have some minimum aberrations. In practical instruments, it was arranged that the limiting defect would be spherical aberration in the objective lens. This allowed the imaging performance to be largely characterized by a single parameter, spherical aberration, and also allowed a certain amount of image simulation and post-experiment analysis to facilitate interpretation of imaging results.