Top panel: luminosity function of the selected PNs in the outer regions of M 87, corrected for colour and detection incompleteness. The error bars show the 1σ uncertainty from counting statistics. Data are binned into 0.3 mag intervals (see Table 5 for numerical values). The solid red line represents the analytical PNLF model for a distance modulus of 30.8, convolved with photometric errors. The black line shows the Lyα LF from Gronwall et al. (2007), scaled to the effective surveyed volume of the M 87 SUB1 and M 87 SUB2 fields, with the shaded area showing the cosmic variance due to Lyα density fluctuations (~20%, see Sect. 3.2). Bottom panel: difference between the PNLF if we had used instead the automatically extracted sources without any final inspection. This plot shows that spurious detections affect mostly the two faintest magnitude bins.