The X-ray diffracted beam intensity changes considerably when a static electric field is applied perpendicularly to a X- or Y-cut LiNbO3 platelet-shaped crystal. Section diffraction images (topographs) recorded using synchrotron radiation in transmission mode reveal the curvature of the lattice planes under such a field. A theoretical analysis based on the equations of electro-mechanical field in a piezoelectric medium together with chemical etching experiments allows us to explain this effect. Tiny inverted ferroelectric domains produced by the applied field exert a strain on the non-inverted bulk leading to the observed curvature of the lattice planes. This approach seems to be valid for other piezoelectric crystals of various symmetry classes.