Depth of field (DoF) in scanning electron microscopes is evaluated at high magnifications where the image resolution is limited by the probe size. The calculation of DoF is made in terms of the information passing capacity of an optical system or entropy of the image estimated along the optical axis near focus. Electron diffraction, source brightness (or source size), and signal-to-noise ratio are taken into account in the calculations of the DoF. The DoF at high magnifications is proportional to R-min(2) where R-min is resolution of the system, and degrades due to the lack of brightness and aberrations. The DoF in semiconductor applications is also estimated.