  Gravitational weak lensing by large scale structures is view as a tool to
probe the bias relation between the mass and the light distributions. It is
explained how a particular statistic can be used to deproject the 2D mass
distribution observed by weak lensing, in order to probe the bias at a given
redshift and a given scale. Dependence with the cosmology is written, and
observational issues are pointed out, like the importance of the redshift
distribution of the galaxies. A signal to noise analysis shows that the scale
dependence of the bias can be measured with a rather modest lensing survey
size.
