The vertical divergence of a photon beam impinging onto a double‐crystal monochromator with asymmetric crystals, and the horizontal divergence in an inclined double‐crystal monochromator, cause an optical distortion of the exit beam. The consequence of this distortion is that a real point‐like source is transformed into a virtual source which has a finite vertical dimension. This may manifest itself on the size of the focus when using conventional focusing optics after the monochromator. For some real sources the height of the corresponding virtual sources are estimated, and it is shown that this effect is more serious in the case of the inclined monochromator. The beam distortions are confirmed also by raytracing simulations. ©1995 American Institute of Physics.