  We derive a compact expression for the second-order correlation function
$g^{(2)} (0)$ of a quantum state in terms of its Wigner function, thereby
establishing a direct link between $g^{(2)} (0)$ and the state's shape in phase
space. We conduct an experiment that simultaneously measures $g^{(2)} (0)$
through direct photocounting and reconstructs the Wigner function via homodyne
tomography. The results confirm our theoretical predictions.
