We summarize the progress on the rest-frame optical spectroscopy of quasars at 3 2.5−5μm . This spectral window has been utilized for detecting redshifted Hα emission lines of our high redshift subsample of quasars. From the calculated emission line widths and luminosities we measured supermassive black hole masses using well calibrated optical mass estimators. Science topics regarding optical based black hole masses at high-z are discussed.