  We discuss about how the relative flux densities among the images of
gravitationally-lensed active galactic nuclei, AGN, can be used to study the
intrinsic AGN variability with high accuracy. Multi-frequency monitoring
observations of resolved gravitational lenses can allow us to detect signals of
very weak variability and also provide information about the jet opacity and
structure. As an example, we investigate the variability of the flux-density
ratio between the two lensed images of the blazar B0218+357, using
dual-frequency cm-wave observations. Similar to our previously reported
submm-wave observations of the lensed blazar PKS1830-211, we observe a clear
chromatic variability, starting short before an increase in the flux-density of
the blazar. The evolution of the flux-density ratios between the blazar images
shows a more clear and rich structure than that of the mere lightcurves of each
individual image. The accuracy in the ratio measurements is allowing us to see
variability episodes in the blazar that are weaker than the natural scatter in
the absolute flux-density measurements. A simple opacity model in the jet is
used to consistently explain the difference between the flux-density-ratio
evolution at the two frequencies.
