Median observed S/N per pixel at the top of the Lyman-α (green), C iv (red), C iii] complex (blue), Mg ii at z > 2 (orange) and Mg ii at z < 2 (yellow) emission lines and in emission-line free regions (black) versus r-PSF magnitude (corrected for Galactic extinction). Two redshift ranges are considered for Mg ii because the emission line is redshifted in regions of the spectra with very different characteristics. At r ~ 22, the median S/N per pixel at the top of the Lyman-α and C iv emission lines is about 4; sufficient to identify most of the quasars. Outside of the emission-line regions, at the same magnitude, the S/N per pixel is about unity.