[M/H] for the different regions in M31 is plotted against the M^* value obtained from fitting their PNLF. The different markers show the different references for the [M/H] measurements – circle: Williams et al. (2017); pentagon: Bernard et al. (2015); triangle: Escala et al. (2020); inverted triangle: Nidever et al. (2020); cross: Tanaka et al. (2010); square: Conn et al. (2016). The theoretical prediction for the values of M^* from Dopita et al. (1992) models is shown with the dotted line. Also shown is the variation of photometric errors in our survey (multiplied by a factor of ten for visual clarity) at the magnitudes corresponding to the given M^* values (as black lines at M^* = −4.74, −3.74 and −2.74 mag).