Aberration Corrected ImagingSince the invention of electron optics during the 1930’s, lens aberrations have limited the achievable spatial resolution to about 50 times the wavelength of the imaging electrons. Positive spherical aberration is unavoidable in a magnetic lens having cylindrical symmetry. The simplest way to compensate for this problem is to employ sets of multipole lenses to produce a negative spherical aberration correction. The difficulty is that these multipole lenses need to be matched in position, orientation and tilt to the original lens, leading to very complex systems of 30-100 optical elements that need to be controlled with parts per million accuracy. Thus aberration correction has only recently becomes possible -- enabled by advances in design of electron optics and by the availability of highly stable electronics and inexpensive computer platforms for control of the many optical elements. 
On the left: Improvement of performance in the STEM with the addition of aberration correction.  The left side shows results for the objective lens transmission as a function of angle ( a shadow map or Ronchigram), the resulting 2Å resolution image, and the frequencies. The signal contrast is now high enough to image individual heavy atoms and their movement as they form a cluster, as shown on the right. 
 M. Haider, S. Uhlemann, E. Schwan, H. Rose, B. Kabius, and K. Urban, Electron microscopy image enhanced, Nature 392, 768-769 (1998).
 P.E. Batson, Niklas Dellby, and O.L. Krivanek, Sub-Angstrom resolution using aberration corrected electron optics, Nature 418, 617-620 (2002).
 P.E. Batson, Challenges and Opportunities of Angstrom-Level Analysis, in Microscopy of Semiconducting Materials, edited by A.G. Cullis (Royal Microscopy Society, Oxford, 2005), pp. 836-837.