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.