B.S.T.J. Briefs: AC Focusing of an Electron Microscope Objective Lens
01 February 1968
AC Focusing of an Electron Microscope Objective Lens By R. D. HEIDENREICH (Manuscript received November 29, 1967) Control of phase contrast in electron microscope images is presently achieved by fine vernier control of the dc objective lens current. An incremental change 8J in lens current produces a corresponding increment 8f in focal length and a focal separation in object space of 8L0. L0 is the distance from lens to conjugate object plane so that AL0 is the axial separation of conjugate object plane and scattering specimen. The defocus phase at the image plane of a ray scattered at angle /3 relative to the axial ray is X = -Lf" 1 ALo/32 with |/C| = 2TT/A.