  The accuracy of mass estimates by gravitational lensing using the thin-lens
approximation applied to Navarro-Frenk-White mass models with a soft truncation
mechanism recently proposed by Baltz, Marshall and Oguri is studied. The
gravitational lens scenario considered is the case of the inference of lens
mass from the observation of Einstein rings (strong lensing). It is found that
the mass error incurred by the simplifying assumption of thin lenses is below
0.5%. As a byproduct, the optimal tidal radius of the soft truncation mechanism
is found to be at most 10 times the virial radius of the mass model.
