  We give an exact series expansion of the Casimir force between plane and
spherical metallic surfaces in the non trivial situation where the sphere
radius $R$, the plane-sphere distance $L$ and the plasma wavelength
$\lambda_\P$ have arbitrary relative values. We then present numerical
evaluation of this expansion for not too small values of $L/R$. For metallic
nanospheres where $R, L$ and $\lambda_\P$ have comparable values, we interpret
our results in terms of a correlation between the effects of geometry beyond
the proximity force approximation (PFA) and of finite reflectivity due to
material properties. We also discuss the interest of our results for the
current Casimir experiments performed with spheres of large radius $R\gg L$.
