Most gravitational lenses are early-type galaxies in relatively low density environments – a “ field ” rather than a “ cluster ” population .
Their average properties are the mass-averaged properties of all early-type galaxies .
We show that field early-type galaxies with 0 < z < 1 , as represented by the lens galaxies , lie on the same fundamental plane as those in rich clusters at similar redshifts .
We then use the fundamental plane to measure the combined evolutionary and K-corrections for early-type galaxies in the V , I and H bands .
Only for passively evolving stellar populations formed at z _ { f } \mathrel { \raise 1.29 pt \hbox { $ > $ } \mkern - 14.0 mu \lower 2.58 pt \hbox { $ \sim$ } } 2 ( H _ { 0 } = 65 km s ^ { -1 } Mpc ^ { -1 } , \Omega _ { 0 } = 0.3 , \lambda _ { 0 } = 0.7 ) can the lens galaxies be matched to the local fundamental plane .
The high formation epoch and the lack of significant differences between the field and cluster populations contradict many current models of the formation history of early-type galaxies .
Lens galaxy colors and the fundamental plane provide good photometric redshift estimates with an empirical accuracy of \langle z _ { FP } - z _ { l } \rangle = -0.04 \pm 0.09 for the 20 lenses with known redshifts .
A mass model dominated by dark matter is more consistent with the data than either an isotropic or radially anisotropic constant M/L mass model , and a radially anisotropic model is better than an isotropic model .