  The location of the beam focus when monochromatic x-ray radiation is
diffracted by a thin bent crystal is predicted by "crystal lens equation". We
derive this equation in a general form valid for Bragg and Laue geometries.
This equation has little utility for diffraction in Laue geometry. The focusing
effect in the Laue symmetrical case is discussed using concepts of dynamical
theory and an extension of the lens equation is proposed. The existence of
polychromatic focusing is considered and the feasibility of matching the
polychromatic and monochromatic focal positions is discussed.

Optics