The maximum efficiency of converting stellar radiation into [O iii] 5007 Å line radiation, (L₅₀₀₇/L_star)^max, of 0.696 M_⊙ (filled circles) and 0.625 M_⊙ models (diamonds) against their metallicity, Z/Z_GD. The upper abscissa gives the corresponding oxygen content relative to hydrogen, ϵ(O)   (= log (O/H) + 12). The numbers along the dotted line segments connecting the 0.696 M_⊙ data points indicate the slope. The inverted parabola (dashed and thick line) is a fit to the predictions of (optically thick) photoionisation models computed by Dopita et al. (1992, Eq. (4.1) therein). The thick portion corresponds to the metallicity range covered by the Dopita et al. models.