... Even an automated search engine able to use an archive metadata (instrument, a filter, exposure time...) is not completely sufficient. ... Here, we show how a ``reversed'' exposure time calculator can be used to efficiently process the database-stored image descriptors of the ESO/Wide Field Imager (WFI) archive, and compute the corresponding limiting magnitudes. ... To compute a limiting magnitude without reprocessing the whole ESO archive images, we chose to use the ESO exposure time calculator. ...