Verifying the Planetary Nebula Luminosity Function Method
  The planetary nebula luminosity function (PNLF) technique for determining distances to galaxies now has been applied to 34 galaxies, including 6 in the Virgo cluster and 3 in the Fornax cluster. Of these, 16 galaxies are late-type or spirals and presumably contain Cepheid variables useful for verifying the PNLF method. For 7 of these galaxies, Cepheid distances exist; the PNLF distances agree with the Cepheid distances within the dispersion of 8% and within a zero-point offset of 1%. In addition, 3 small groups were studied (NGC 1023, Leo I, and Coma I) where both spiral and elliptical distances were obtained to investigate the magnitude of any systematic dependence on spiral versus elliptical Hubble type. None was found. Since the PNLF method agrees well with the Cepheid system, and there is no measurable dependence on Hubble type, it follows that PNLF distances to the ellipticals in Virgo and Fornax also are on the Cepheid scale. This conclusion is strengthened by the Cepheid distances to several Virgo galaxies and the recent determination of a Cepheid distance to Fornax. 
George Jacoby (National Optical Observatories)