For a non-negative separable random field Z(t),
$t\in \mathbb{R}^d$, satisfying some mild assumptions, we show that
$ H_Z^\delta =\lim_{{T} \to \infty} ({1}/{T^d}) \mathbb{E}\{{\sup_{ t\in [0,T]^d \cap \delta \mathbb{Z}^d } Z(t) }\} <\infty$ for
$\delta \ge 0$, where
$0 \mathbb{Z}^d\,:\!=\,\mathbb{R}^d$, and prove that
$H_Z^0$ can be approximated by
$H_Z^\delta$ if
$\delta$ tends to 0. These results extend the classical findings for Pickands constants
$H_{Z}^\delta$, defined for
$Z(t)= \exp( \sqrt{ 2} B_\alpha (t)- \lvert {t} \rvert^{2\alpha})$,
$t\in \mathbb{R}$, with
$B_\alpha$ a standard fractional Brownian motion with Hurst parameter
$\alpha \in (0,1]$. The continuity of
$H_{Z}^\delta$ at
$\delta=0$ is additionally shown for two particular extensions of Pickands constants.