Depth of field in phase-contrast transmission electron microscopy

The depth of field (DoF) is an important parameter in transmission electron microscopy (TEM) when considering inhomogeneous samples, tomographic schemes, or in-situ experiments. A large DoF enhances the robustness of the experiment against tight alignment demands and drift issues. The phase-contrast transfer in TEM is reviewed and updated with special regard to the correct derivation and quantification of the DoF for relevant single-transfer-band imaging like with Scherzer or Lentzen conditions. Consequences towards the optimisation of the imaging conditions are discussed. Additionally, the defocus-dependent information limit is treated as a different type of DoF. One result of this work is to prefer the original Scherzer defocus over the Scherzer defocus taught today if a large DoF matters in non-aberration-corrected instruments. In aberration-corrected instruments, the Lentzen conditions are well suited for a maximum DoF if one adapts the desired maximum spatial frequency accordingly, and higher-order spherical-aberration correction might help at lower electron acceleration voltages.