Abstract: Long term photometric light curves of strongly lensed quasars, as currently obtained by the COSMOGRAIL program and as will be obtained with Rubin-LSST, can be used both to measure the Hubble constant and to study quasar structure on scales as small as light days. I will describe how the method has been implemented, with its unique advantages, degeneracies and limitations. One particular object attracted much attention lately as the microlensed source is likely a milli-parsec separation supermassive black hole that can be detected by Pulsar Timing Arrays.