  Gravitational lensing is generally treated in the geometric optics limit;
however, when the wavelength of the radiation approaches or exceeds the
Schwarzschild radius of the lens, diffraction becomes important. Although the
magnification generated by diffractive gravitational lensing is well
understood, the astrometric signatures of diffractive microlensing are first
derived in this paper along with a simple closed-form bound for the astrometric
shift. This simple bound yields the maximal shifts for substellar lenses in
solar neighbourhood observed at 20~GHz, accessible to high sensitivity, high
angular resolution radio telescopes such as the proposed Square Kilometre Array
(SKA).
