The incident electron beam can be considered as made of a set of incident rays within an incident cone. In two-beam condition, the resulting pattern located in the back focal plane consists of two disks but not of two spots, namely the transmitted and hkl diffracted disks as shown in Figure 3885a (a). Given the strength of the objective lens is unchanged, when the defocus is changed from zero to Δh by adjusting the height of the TEM sample, the transmitted and diffracted beams in the diffraction pattern in the back focal plane will be changed from Figure 3885a (a) to (b). In this case, the diffraction spots in the image plane also move. Assuming the strengths of the other lenses in the TEM system are not changed, the diffraction pattern on the TEM screen or detector will be very different from the cases in Figure 3885a (a) to (b). If you are working on TEM-imaging mode, the TEM image will also be very different.
Figure 3885a. TEM optics under conditions of focused (a) and defocused (b) convergent incident electron beams.
Figure 3885b shows an underfocus TEM image of a (001) SrTiO3 crystal with an incident probe focused on the specimen. The caustic curve is also indicated in the figure.
Figure 3885b. An underfocus TEM image of a (001) SrTiO3 crystal with an incident probe focused on the specimen.
Adapted from 
Although Figure 3885b appears to be convergent-beam electron diffraction patterns, it is TEM images at a magnification of 1E5 with a defocus of about +5 µm.
Koji Kimoto, Kazuo Ishizuka, Nobuo Tanaka, Yoshio Matsui, Practical procedure for coma-free alignment using
caustic figure, Ultramicroscopy 96 (2003) 219–227.