Planetary Nebulae (PNe) in external galaxies are mostly regarded either as tracers of the gravitational potential as indicators for the distance of their galactic hosts, with the latter advantage owing to the almost universal -- though not fully understood -- shape of the PNe luminosity function (PNLF, generally in the [OIII]5007 emission). Yet extra-galactic PNe can also be used as probes of their parent stellar population and understanding in particular the origin of the PNLF is a puzzle that, once solved, promises to reveal new clues on the late stages of stellar evolution and on the formation of PNe themselves. I will show how integral-field spectroscopy allows to detect PNe in the optical regions of galaxies to flux levels otherwise inaccessible to standard narrow-band photometry. This allows to trace the PNLF up to a larger completeness limit and in the same galactic regions where the properties of stellar populations can be well characterised, thus making it possible to understand how the shape of PNLF relates to the parent stellar population of PNe. I will present results based on the detailed analysis of SAURON data for M32 and M31, showing for the first time that the PNLF is in fact not universal, and the future prospect of this field with next generation integral-field units such as MUSE on VLT.