Lawrence Berkeley National Laboratory

The change in focus of a high-resolution electron microscope is generally assumed to be linear with change in objective lens current. Thus the defocus step size should be constant for a constant step in lens current. Measurements on the LBNL One-Angstrom Microscope show that the step size increases with increasing underfocus (reduced lens current). Differentiation of the best-fit quadratic shows that the defocus step size varies linearly as defocus changes.