The Planetary Nebula Luminosity Function (PNLF) technique for determining distances to galaxies is described in Jacoby et al. (1992). Briefly, the method relies on the constancy of the [OIII] 5007 luminosity at which the population of high luminosity planetary nebulae (PN) declines rapidly. While the number of PN decreases monotonically with increasing luminosity, there exists a luminosity beyond which PN are not observed at all. This limiting luminosity is a consequence of several factors (Jacoby (1989), Kaler & Jacoby (1991), Méndez et al. (1993), Han et al. (1994), Stanghellini (1995), Richer et al. (1997)), the most important being the independence between progenitor age (or initial mass) and the final white dwarf (or central star) mass over reasonable age ranges (e.g. 3-11 Gyrs). For the great majority of progenitors and, therefore, for normal populations the critical PN luminosity varies by less than 0.1 mag.
The PNLF technique is one of the few that has been demonstrated to yield consistent results in both elliptical and spiral galaxies at distances up to 20 Mpc. Consequently, it offers a path to unify the population I distances (e.g. Cepheids) with population II objects (e.g. elliptical galaxies). The former are needed for calibration while the latter are needed to define distances to rich clusters. Other promising cross-population indicators include SN Ia when corrected for decline rate variations (Hamuy et al. (1996)), surface brightness fluctuations (SBF), and globular cluster luminosity functions (GCLF).
This paper describes recent results in which the PNLF has been extended to late-type galaxies in order to improve the zero-point calibration, and new observations in M87 to assess the legitimacy of the arguments posed against the use of the PNLF by Bottinelli et al. (1991) and Tammann (1993).