We present the findings of our weak lensing study of a sample of 116 CNOC2 galaxy groups .
The lensing signal is used to estimate the mass-to-light ratio of these galaxy groups .
The best fit isothermal sphere model to our lensing data has an Einstein radius of 0 . ^ { \prime \prime } 88 \pm 0 . ^ { \prime \prime } 12 , which corresponds to a shear-weighted velocity dispersion of 245 \pm 18 km s ^ { -1 } .
The mean mass-to-light ratio within 1 h ^ { -1 } Mpc is 185 \pm 28 hM _ { \odot } /L _ { B \odot } and is independent of radius from the group center .

The signal-to-noise ratio of the shear measurement is sufficient to split the sample into subsets of “ poor ” and “ rich ” galaxy groups .
The poor galaxy groups were found to have an average velocity dispersion of 193 \pm 38 km s ^ { -1 } and a mass-to-light ratio of 134 \pm 26 hM _ { \odot } /L _ { B \odot } , while the rich galaxy groups have a velocity dispersion of 270 \pm 39 km s ^ { -1 } and a mass-to-light ratio of 278 \pm 42 hM _ { \odot } /L _ { B \odot } , similar to the mass-to-light ratio of clusters .
This steep increase in the mass-to-light ratio as a function of mass , suggests that the mass scale of \sim 10 ^ { 13 } M _ { \odot } is where the transition between the actively star-forming field environment and the passively-evolving cluster environment occurs .
This is the first such detection from weak lensing .