Predicted S/N of Hα, Hβ, [O III] and [N II] lines in function of the S/N in the photometry. For a given S/N in the photometry, each point represent the mean S/N obtained in the line for 500 SDSS galaxies in the interval (color-coded) γ < logEW_SDSS < γ + 0.1 with γ ranging from 0.8 to 2.5 for Hα, from 0.8 to 2.2 for [O III], from 0.8 to 1.8 for Hβ and from 0.8 to 1.8 for [N II]. Errors bars indicate the mean absolute deviation. Dashed red line represents Eq. (5) for EW = 10 Å.