  We present a concise derivation of geometric optics in the presence of
axionic fields in a curved space-time. 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.
