Image formation basics
The TEM images are formed in two stages:
Stage A is the scattering of an incident electron beam by a specimen. This scattered radiation passes through an objective lens, which focuses it to form the primary image.
Stage B uses the primary image obtained in stage A and magnifies this image using additional lenses to form a highly magnified final image.
In the process of forming the primary image the objective lens produces a diffraction pattern at its back focal plane. The diffraction pattern is a Fourier transform of the scattered electron wave. The primary image is the Fourier transform of the diffraction pattern.
This two-step process forms the basis of image formation during high-resolution transmission electron microscopy (HRTEM).
The high-resolution image is, in effect, an interference pattern of the beams formed at the back focal plane of the objective lens.