  We use an exact general relativistic model structure within an FRW
cosmological background based on a LTB metric to study the gravitational
lensing by a cosmological and dynamical structure. Using different density
profiles for the model structure, the deviation angle and the time delay
through the gravitational lensing has been studied by solving the geodesic
equations. The results of these exact calculations have been compared to the
thin lens approximation. We have shown that the result for the thin lens
approximation based on a modified NFW density profile with a void before going
over to the FRW background matches very well with the exact general
relativistic calculations. However, the thin lens approximation based on a
normal NFW profile does differ from the exact relativistic calculation. The
difference is more the less compact the structure is. We have also looked at
the impact of our calculation on the observational interpretation of arcs in
the case of strong lensing and also the reduced shear in the case of weak
lensing. No significant difference has been seen in the data available.
