Colour-magnitude diagram for a single field showing all the detected sources (brighter than the limiting magnitude) in black, the colour-selected sources in blue, and identified PNe in red. The 99% and 99.99% limits for the continuum sources are shown in blue, while the region beyond the 50% completeness limit is shown in grey. The histogram (in logarithmic scale) shows clearly the number of PNe recovered as a function of colour with all the detected sources (brighter than the limiting magnitude) in black, the colour-selected sources in blue, and identified PNe in red.