We propose the use of the H II region luminosity function (LF) as a refined powerful standard candle for extragalactic distances. We have found evidence of a change in the properties of H II regions in spirals at a fixed luminosity in Hα which we term the Strömgren luminosity (LHα = 3.39×1038 erg s-1). The key piece of evidence is the presence in the Hα H II region LF of a change in gradient and a local sharp peak. The invariance and high luminosity of the change measured in the LF was proposed in previous studies as a possible secondary standard candle, but it is calibrated here for the first time using the LFs of NGC 925, NGC 4535 and M100, whose Cepheid distances have been determined in the HST Extragalactic Distance Scale Key Project. Using this method we derive a value of H0 of 73±7 km s-1 Mpc-1.