We study the mass-to-light ratio of galaxy systems from poor groups to rich clusters and present for the first time a large database for useful comparisons with theoretical predictions. We extend a previous work, where Bj band luminosities and optical virial masses were analyzed for a sample of 89 clusters. Here we also consider a sample of 52 more clusters, 36 poor clusters, seven rich groups, and two catalogs, of ~500 groups each, recently identified in the Nearby Optical Galaxy sample by using two different algorithms. We obtain the blue luminosity and virial mass for all systems considered. We devote a large effort to establishing the homogeneity of the resulting values, as well as to considering comparable physical regions, i.e., those included within the virial radius. By analyzing a fiducial, combined sample of 294 systems we find that the mass increases faster than the luminosity: the linear fit gives M~L1.34+/-0.03B, with a tendency for a steeper increase in the low-mass range. In agreement with the previous work, our present results are superior owing to the much higher statistical significance and the wider dynamical range covered (~1012-1015 Msolar). We present a comparison between our results and the theoretical predictions on the relation between M/LB and halo mass, obtained by combining cosmological numerical simulations and semianalytic modeling of galaxy formation.
