image This can be seen very simply: Imagine a lens situation like the one displayed in Figure  2 . If now all length scales are reduced by a factor of two and at the same time all masses are reduced by a factor of two, then for an observer the angular configuration in the sky would appear exactly identical. But the total length of the light path is reduced by a factor of two. Now, since the time delay between the two paths is the same fraction of the total lengths in either scenario, a measurement of this fractional length allows to determine the total length, and hence the Hubble constant, the constant of proportionality between distance and redshift.