  Ultrafast X-ray diffraction by the LCLS free-electron laser has been used to
probe Charge Density Wave (CDW) systems under applied external currents. At
sufficiently low currents, CDW wavefronts bend in the direction transverse to
the 2k$_F$ wave vector. We show that this shear effect has the ability to focus
or defocus hard X-ray beams, depending of the current direction, making it an
electronic lens of a new kind, tunable at will from the Fraunhofer to the
Fresnel regime. The effect is interpreted using the fractional Fourier
transform showing how the macroscopic curvature of a nanometric modulation can
be beneficially used to modify the propagation of X-ray beams.
