Comparison between Hα line widths measured using SDSS and TNG/NTT. The top panel shows the observed values, while the bottom panel shows the values obtained after subtracting in quadrature the instrumental resolution. The average resolution for the three instruments is shown by colored dotted lines for reference. Magenta circles mark 12 particularly compact galaxies, for which over 30 percent of r-band flux from a 10” aperture is contained within the 3” SDSS fiber. The gray dashed line shows the one-to-one relation. We show only galaxies in which Hα is detected with an S/N greater than 3 and for which the uncertainty on the best-fit line width does not exceed the line widths itself. We chose these thresholds to exclude galaxies in which the line width cannot be determined accurately.