Gravitational lensing allows to determine the Hubble constant directly. In this case the lens is a galaxy and the source a quasar, which should be mapped at least twice. When one of the images changes its brightness, the other one does, too - but not exactly at the same time. A delay is caused by two effects. One is due to the difference of the length of light-paths, and the other is due to the influence of the gravitational lens on the run-time of light-rays. If the size and the mass-distribution of the lens is known, one can determine from the measured delay of changes in brightness directly the total light-travel time from the quasar, hence its distance.
In this way, for the first time in 1995 the Hubble constant was determined - independent from previous methods - at the double quasar Q0957+561.