Geometric optics in the presence of axionlike particles in curved spacetime

We present a concise derivation of geometric optics in the presence of axionic fields in a curved spacetime. Whenever light can be described via geometric optics (the eikonal approximation), the only difference to the situation without axionic field is the phenomenon of achromatic birefringence. Consequently, redshift of light and distance estimates based on propagating light rays, as well as shear and magnification due to gravitational lensing are not affected by the interaction of light with an axionic field.