To discuss the visibility of defects in topographic images according to theory, consider the exemplary case of a single dislocation: It will give rise to contrast in topography only if the lattice planes involved in diffraction are distorted in some way by the existence of the dislocation. This is true in the case of an edge dislocation if the scattering vector of the Bragg reflection used is parallel to the Burgers vector of the dislocation, or at least has a component in the plane perpendicular to the dislocation line, but not if it is parallel to the dislocation line. In the case of a screw dislocation, the scattering vector has to have a component along the Burgers vector, which is now parallel to dislocation line. As a rule of thumb, a dislocation will be invisible in a topograph if the vector product
is zero.
(A more precise rule will have to distinguish between screw and edge dislocations and to also take the direction of the dislocation line  into account – see e.g. [http://www.msel.nist.gov/practiceguides/SP960_10.pdf].)
If a defect is visible, often there occurs not only one, but several distinct images of it on the topograph. Theory predicts three images of single defects: The so-called direct image, the kinematical image, and the intermediary image.
For details see e.g. (Authier 2003).