Left panel: logarithm of the [O III] emission line flux (in units of 10^-17   erg   s^-1   cm^-2) versus the k-corrected i-band magnitude within the SDSS fiber, both quantities corrected for galactic absorption. For clarity, upper limits in the line flux are not reported. The dashed red line corresponds to a constant ratio between the two quantities. Right panel: histogram of the residuals from the median line. The dashed histogram is the contribution of upper limits. The solid line is a Gaussian distribution with a width of 0.19 dex. Bottom: residuals from the median vs. redshift.