  It is known that a relative translational motion between the deflector and
the observer affects gravitational lensing. In this paper, a lens equation is
obtained to describe such effects on actual lensing observables. Results can be
easily interpreted in terms of aberration of light-rays. Both radial and
transverse motions with relativistic velocities are considered. The lens
equation is derived by first considering geodesic motion of photons in the
rest-frame Schwarzschild spacetime of the lens, and, then, light-ray detection
in the moving observer's frame. Due to the transverse motion images are
displaced and distorted in the observer's celestial sphere, whereas the radial
velocity along the line of sight causes an effective re-scaling of the lens
mass. The Einstein ring is distorted to an ellipse whereas the caustics in the
source plane are still point-like. Either for null transverse motion or up to
linear order in velocities, the critical curve is still a circle with its
radius corrected by a factor (1+z_d) with respect to the static case, z_d being
the relativistic Doppler shift of the deflector. From the observational point
of view, the orbital motion of the Earth can cause potentially observable
corrections of the order of the microarcsec in lensing towards the
super-massive black hole at the Galactic center. On a cosmological scale,
tangential peculiar velocities of cluster of galaxies bring about a typical
flexion in images of background galaxies in the weak lensing regime but future
measurements seem to be too much challenging.
