CMD for a simulated PN population in the M 87 SUB1 field with intrinsic magnitudes 23 ≤ m_n ≤ m_lim,n (crosses). Because of photometric errors, our selection criteria would exclude 28.3% of the input sample (blue crosses). The remaining 71.7% would be selected as PNs (red crosses). Dots surrounded by green squares and cyan triangles are emission objects in common between our sample and the PN samples selected by Ciardullo et al. (1998) and Feldmeier et al. (2003), respectively. Solid lines as in Fig. 3.