IMF and velocity-dispersion effects on optical spectroscopic absorption features

We explore the behaviour of the most popular spectroscopic absorption featu res in the optical region as a function of the initial mass function (IMF), while taking into account the spectral resolution and galaxy velocity disp ersion. We conclude that the IMF produces a negligible effect on the Lick i ndices contained in the wavelength interval lambda lambda 3800-5500 Angstro m, except the Mg-1 index, for which the IMF could be as important as the me tallicity. We also show that the IMF is irrelevant for most of the Rose (19 94) indicators, except for various indices that show a non-negligible sensi tivity. In contrast to the Lick indices, some of the Rose indicators vary t heir age-metallicity-IMF trends as a function of the spectral resolution.