Photon trajectories on the equatorial plane of the Kerr space-time with a = 0.8. We obtained the trajectories by integrating backward in time from a distant observer on the x axis. The solid black lines correspond to the prograde and retrograde circular photon orbits, obtained for the critical values, b_c,±, of the impact parameter. Photons with impact parameters between b_c,+ and b_c,− are bound to the black hole and cross the horizon (colored solid lines). Outside this interval photons escape to infinity (gray lines). Due to frame dragging the two critical impact parameters are not symmetrical with respect to the central black hole, which ultimately causes the asymmetry of the Kerr shadow.