For lensing galaxies we introduce a formal velocity dispersion \sigma _ { \textrm { \tiny { lens } } } , based on enclosed mass and the virial theorem .
This is calculated from an ensemble of pixelated lens models , and found to be fairly model independent .
A sample of 18 well-known early-type lensing galaxies and two clusters is found to be consistent with \sigma _ { \textrm { \tiny { lens } } } = \sigma _ { \textrm { \tiny { obs } } } .
Both the early-type lensing galaxies and the clusters can thus be determined as being virialized .
In a second step we calculate the I-band luminosity and the total mass content for the sample of lensing galaxies , which enables us to analyze the mass-to-light relation L \propto M ^ { \alpha } .
We determine \alpha = 0.70 \pm 0.08 , excluding constant M / L and consistent with previous studies of the Fundamental Plane .
Additionally we verify that this relation does not extrapolate to clusters , which have a much higher M / L .
The sample used for this analysis comprises 9 lensing galaxies from the Sloan Lens ACS Survey ( SLACS ) and another 9 from the CfA-Arizona Space Telescope LEns Survey ( CASTLES ) as well as the lensing clusters ACO 1689 and ACO 2667 .