We explore the behaviour of the most popular spectroscopic absorption features in the optical region as a function of the initial mass function (IMF), while taking into account the spectral resolution and galaxy velocity dispersion. We conclude that the IMF produces a negligible effect on the Lick indices contained in the wavelength interval λ 3800-5500 Å, except the Mg_1 index, for which the IMF could be as important as the metallicity. We also show that the IMF is irrelevant for most of the Rose (1994) indicators, except for various indices that show a non-negligible sensitivity. In contrast to the Lick indices, some of the Rose indicators vary their age-metallicity-IMF trends as a function of the spectral resolution.