The effect of variations in objective focal length on electron microscope performance

Standard specimens were examined in a transmission electron microscope with a top entry stage. Using a range of objective focal lengths, measurements of contrast, resolution and magnification range were made. Focal lengths up to 14 mm were examined. The contrast was found to increase with increasing focal length, being twelve and a half times greater at 14 mm focal length than at the normal high resolution mode of 2 mm focal length. Resolution, determined by the fresnel fringe method decreased from 0-3 nm to 3-5 nm. The photographic image mangnification was reduced from X 1000-X500,000 at 2 mm focal length to X100-X50,000 at 14 mm focal length.