  The [O III] 5007 planetary nebula luminosity function (PNLF) occupies an
important place on the extragalactic distance ladder. Since it is the only
method that is applicable to all the large galaxies of the Local Supercluster,
it is uniquely useful for cross-checking results and linking the Population I
and Population II distance scales. We review the physics underlying the method,
demonstrate its precision, and illustrate its value by comparing its distances
to distances obtained from Cepheids and the Surface Brightness Fluctuation
(SBF) method. We use the Cepheid and PNLF distances to 13 galaxies to show that
the metallicity dependence of the PNLF cutoff is in excellent agreement with
that predicted from theory, and that no additional systematic corrections are
needed for either method. However, when we compare the Cepheid-calibrated PNLF
distance scale with the Cepheid-calibrated SBF distance scale, we find a
significant offset: although the relative distances of both methods are in
excellent agreement, the PNLF method produces results that are systematically
shorter by about 15%. We trace this discrepancy back to the calibration
galaxies and show how a small amount of internal reddening can lead to a very
large systematic error. Finally, we demonstrate how the PNLF and Cepheid
distances to NGC 4258 argue for a short distance to the Large Magellanic Cloud,
and a Hubble Constant that is about 8% larger than that derived by the HST Key
Project.
